Callovian (Jurassic) 

Ammonites from the 

United States and 

Alaska

Part 1. Western Interior United States

GEOLOGICAL SURVEY PROFESSIONAL PAPER 249-A

Callovian (Jurassic) 

Ammonites from the 

United States and 

Alaska

Part 1. Western Interior United States 

By RALPH W. IMLAY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 249-A

Descriptions and illustrations of 

cephalopods of Late Jurassic age

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1953
UNITED STATES DEPARTMENT OF THE INTERIOR

Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office

Washington 25, D. C.
CONTENTS

Page

Abstract -—--—---—-——————_—————__________.._____.„__.______ 1

Introduction --——————————————.._____________._„._.„___ 1

Biologic analysis ——————.——__——._____.___.._____„_„_____ 1

Stratigraphic summary ———————._.__——____.____________ 4

Faunal zones and correlations————..__————_.._——— 5

Arcticoceras codyense zone..————.._______....——_„ 5

Gowericeras costidensum zone____—._...„——_.——_——— 7

Gowericeras subitum zone————_-.........._..._„„........ 7

Kepplerites tychonis zone.__————————_———. 7

Kepplerites mcleami zone.—————„___„..—.............. 7

Comparisons with other faunas————____........„—....„.. 8

Ecologic considerations —————————_——.__..._.... 8

Sources of sediment———_——.._ _„__..—_———._.. 8

	Page

Ecologic considerations—Continued

Conditions of deposition—— _.____.__———-——.——— 9

Conglomerate —...— ——————————_.., 9

Sand ——_———————————————— — 9

Marine siltstone and shale.—————-—_——— 10

Limestone -——————_.-_————————.——— 10

Other types of sediment———————————— 10

Ammonite distribution and associations—————— 11

Geographic distribution - - ———————————————— 14

Summary of results--------———————————————— 17

Systematic descriptions ———.—_.————————— 18

References ———————————————————— ———. 34

Index -.-—————————.———————-—-—--——— 37

ILLUSTRATIONS

	[Plates 1-24 follow index]

PLATE 1. Xenocephalites and Lilloettia

2-4. Arcticoceras

5. Cosmoceras and Arcticoceras

6,7. Cadoceras

8, 9. Cadoceras and Cosmoceras

10-13. Cadoceras

14. Cosmoceras, Cadoceras, and Kepplerites

15-18. Kepplerites

19. Kepplerites and Grossouvria

20. Kepplerites

21,22. Gowericeras

23. Gowericeras and Procerites

24. Grossouvria and Procerites

FIGURE 1. Index map showing Callovian localities in the western interior of the United States2. Lower Callovian paleogeography of the Boreal region ...——.__ - _____......——__._

Page

12

13

TABLES

TABLE 1. Stratigraphic distribution of Callovian ammonite species in the western interior of the United States—.——. 6

2. Correlation of Jurassic formations in the western interior of the United States...——————————Pacing 8

3. Geographic distribution of the Callovian ammonities in the western interior of the United States ——Facing 12

GALLOVIAN (JURASSIC) AMMONITES FROM THE 

UNITED STATES AND ALASKA

PART 1. WESTERN INTERIOR UNITED STATES 

By RALPH W. IMLAY

ABSTRACT

Early Late Jurassic ammonites have been obtained from 

the Rierdon formation of Montana and the lower part of the 

Sundance formation of Wyoming and South Dakota. These 

ammonites represent only the lower part of the Callovian 

stage and correspond with the northwest European zones of 

Macrocephalites macrocephalus, Proplanulites koenigi and 

Sigaloceras calloviense. Possibly the highest ammonite-bearing 

beds are as young as the Cosmoceras jason zone, although 

most of the evidence is against such a correlation.

Careful stratigraphic collecting has demonstrated the existence of five ammonite zones and their positions with respect to 

various lithologic units. The basal zone, characterized by 

Arcticoceras .codyense, has been identified at many localities 

in the basal oolitic beds of the Sundance formation in Wyoming and South Dakota and in the lowermost beds of the 

Rierdon formation in Montana. It is correlated with the 

European zone of Macrocephalites macrocephalus on the basis 

of its stratigraphic position beneath beds containing abundant 

Gowericeras and because it contains numerous Cadoceras that 

are not older than the Callovian. The immediately overlying 

beds in central and nothwestern Montana have furnished a 

densely ribbed ammonite, called Gowericeras costidensum. 

Above, near the middle of the Rierdon formation, follow beds 

containing abundant Gowericeras and Cadoceras and characterized by the species Gowericeras subitum. These beds containing Gowericeras are correlated with the northwest European zone of Proplanulites koenigi because of the abundance 

of Gowericeras and the highest occurrence of Procerites. The 

overlying beds which contain Kepplerites tychonis, are correlated with the lower part of the Sigaloceras calloviense zone 

of Europe because the same species occurs in Alaska and East 

Greenland in association with Cosmoceras (Gulielmiceras). 

The highest beds of the Rierdon formation in Montana contain many Kepplerites, of which K. mclearni has been designated the zonal index. Correlation with the Sigaloceras calloviense zone of northwest Europe is indicated by the presence 

of Cosmoceras (Gulielmiceras) and C. (Gulielmites), by the 

abundance of Kepplerites with flattened venters, and by the 

occurrence of similar forms in Alaska in beds containing 

Gowericeras and Paracadoceras.

The Callovian ammonites of the western interior sea show 

greater resemblances to those of East Greenland than to 

those of southwestern Alaska. This probably reflects differences in depth of water, as all available evidence shows that 

the sea in the area of southwestern Alaska was fairly deep 

and bordered mountainous shores, whereas the seas of the 

western interior and of Greenland were very shallow and 

bordered low shores.

INTRODUCTION

Studies of the Callovian ammonites of the western 

interior region were begun by the writer in the summer of 1944 and were nearly completed the follovdng 

year except for photographic work. The delay in 

publication was beneficial in several ways: important collections and valuable field data were added 

in Montana and Wyoming, the Callovian ammorites 

of Alaska were studied thoroughly, and a much better understanding was obtained of comparable 

faunas of Europe, Asia, and South America. In particular, the many months spent in the field measuring sections, tracing lithologic units, and collecting 

fossils made possible definite conclusions concerning 

faunal zones and stratigraphic correlations that 

should be useful to mapping parties.

Most of the fossils described herein have been collected since 1938, but some were collected as early as 

1885 in the Yellowstone National Park area. The 

principal collectors were T. W. Stanton in 1911 and 

1921, A. J. Collier in 1921 and 1923, D. A. Andrews 

and W. G. Pierce in 1935, J. B. Reeside, M. M. Kn*chtel and S. W. Hobbs in 1938, C. F. Deiss and R. M. 

Garrells in 1940, Josiah Bridge in 1941, William Cobban in 1944 and 1946, R. W. Imlay from 1944 to 

1951, Oscar Mueller in 1944 and 1946, and J. D. 

Love, Theodore Botinelly and George Pipiringo^ in 

1946.

BIOLOGIC ANALYSIS

The Callovian ammonites from the western interior of the United States include 482 specimens 

that are specifically identifiable. Among these the 

Macrocephalitidae is represented by 11, the Cardioceratidae by 230, the Cosmoceratidae by 221, and the 

Perisphinctidae by 20. In the Cardioceratidae, Arcticoceras comprises 20 percent and Cadoceras 28 percent of the total number of ammonites present. In 

the Cosmoceratidae, Kepplerites comprises 15 percent, Gowericeras 23 percent, and Cosmoceras 8
CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

percent of the total. In number of species the Macrocephalitidae is represented by 4, the Cardioceratidae 

by 10, the Cosmoceratidae by 17, and the Perisphinctidae by 7. The ammonites are distributed among 

10 genera and subgenera, and 37 species and varieties. Nineteen species and 1 variety are described 

as new, 5 have been described previously from western interior collections, 1 is identified with a common 

boreal species, 1 is compared with a boreal species, 

1 is compared with a Mexican species, 2 are compared with European species, and 7 are not named 

because they are poorly preserved.

The Macrocephalitidae is represented by Lilloettia 

and Xenocephalites. These have been found only in 

the Kepplerites mclearni zone at the top of the Rierdon formation in Montana. Lilloettia has been reported previously from Alaska and British Columbia 

(Crickmay, 1930, pp. 60, 62). In Alaska Lilloettia is 

associated with other ammonites that indicate correlation with the middle Callovian zones of Sigaloceras 

calloviense and Cosmoceras jason in northwest Europe. Xenocephalites occurs in southwestern Alaska, 

in East Greenland (Spath, 1932, p. 44, pi. 14, figs. 

4 a-d), in southern Mexico (Burckhardt, 1927, p. 33, 

pi. 16, figs. 4-9), and South America (Stehn, 1924, 

pp. 86-92, pi. 1, figs. 3-6). It ranges from the Bathonian into the Callovian, but probably not higher 

than the S. calloviense zone. Both of these genera 

are migrants from the Pacific region and are unknown in Europe. Their morphologic characteristics 

and generic relationships are discussed in detail by 

the writer in a paper dealing with the Callovian ammonites from Alaska (U. S. Geol. Survey Prof. Paper 

249-B) and need not be repeated here.

The Cardioceratidae is represented by Arcticoceras and Cadoceras. Arcticoceras includes at least 

six species that show considerable range in ornamentation and in whorl shape. All species are characterized by a narrowly rounded to subtrigonal venter, by 

an extremely small umbilicus without an umbilical 

edge, and in the immature forms by forwardly inclined ribs that bifurcate near the middle of the 

flanks and arch forward noticeably on the venter. 

Most species develop a nearly smooth body chamber 

and some become smooth at an early age. The aperture is prolonged ventrally in a long lappet. There 

is very slight or no umbilical enlargement at the anterior end of the adult body chamber. On some specimens the suture line of the last septate whorl 

develops broad lobes and saddles and the second 

lateral lobe may become bifid.

The genus is represented by the following species:

A. ishmae (Keyserling), 1846, p. 331, pi. 20, figs.

8-10; Sokolov, 1912, pp. 15, 49, pi. 1, fig. 1, pi. 3,

fig. 12, text. fig. 2; Spath, 1932, pi. 15, figs. 7a, b. 

A. kochi Spath, 1932, p. 53, pi. 12, fig. 1, pi. 13, figs.

4, 5, pi. 14, figs. 1-3, pi. 15, figs. 1, 4-6. 

A. michaelis Spath, 1932, p. 56, pi. 13, figs. 3a, b. 

A. ishmae Madsen (not Keyserling), 1904, p. 191, pi.

8, fig. 7.

A. codyense Imlay, 1948, p. 20, pi. 6, figs. 4, 6, 8. 

A. henryi (Meek and Hayden), 1865, p. 122. pi. 5,

figs. 9 a-c.

A. rierdonense Imlay. 

A. n. sp. ind. (see pi. 3, figs. 13, 15, 17). 

A, crassicostatum Imlay. 

A. loveanum Imlay.

Possibly the list should include Cadoceras subpatruum Nikitin (1885a, p. 58, pl.'ll (13), fig?. 58), 

whose adult whorl greatly resembles that of the 

adult A. henryi (Meek and Hayden). Of the species 

listed, A. codyense and A. henryi lose their ribbing 

earliest; A. ishmae, A. kochi, and A. crassicortatum 

have the coarsest ribbing; A. codyense has the most 

nearly trigonal whorl section. The whorl shape of 

A. codyense suggests a relationship to the keeled 

Chamoussetia R. Douville (1912, p. 19), of the lower 

Callovian, or to the middle and upper Callovian 

Longaeviceras Buckman (1919, p. 121b, pi. 121a), 

which has keeled inner whorls. However, mo^t species of Arcticoceras show greater resemblances to 

the slightly older Arctocephalites, as pointed out by 

Spath (1932, p. 52). The latter genus is distinguished from Arcticoceras by having a wider umbilicus, lower and more rounded whorl sections and 

nearly radial ribbing. Arcticoceras is placed in the 

Cardioceratidae rather than in the Macrocephalitidae because of its compressed form, narrowed venter, and forwardly inclined ribbing of its inn°.r and 

intermediate whorls. The stratigraphic position of 

Arcticoceras at the very base of the Callovian in the 

western interior and boreal regions suggests the possibility that it is ancestral to the similar but slightly 

younger Pseudocadoceras, Longaeviceras, ard certain compressed forms of Cadoceras of which C. 

stenolobum (Keyserling) (1846, p. 329, pi. 20, fig. 7, 

pi. 22, figs. 13, 14) is typical.

The genus Cadoceras will be discussed by the 

writer in detail in a paper dealing with the Callovian 

ammonites of southwestern Alaska and will V?. subdivided into seven groups or subgenera based mainly 

on the varied and prolific Alaskan forms. It is of 

considerable interest therefore that all the species 

of Cadoceras from the western int,erior of the United 

States belong to a group that has not been fcrmd in 

Alaska but occurs in East Greenland and central
WESTERN INTERIOR UNITED STATES

Russia in beds that probably correspond to the European zone of Sigaloceras calloviense. This group 

is characterized by a narrow tubular umbilicus, moderately to broadly rounded whorls, an evenly rounded 

umbilical edge, a lack of tubercles or tubercles 

weakly developed on inner whorls only, and a globose, 

smooth body chamber. The adults are much larger 

than in any other group of Cadoceras (see pis. 7, 10, 

12). This group is represented by the following 

species:

C. franciscits Spath, 1932, p. 74, pi. 20, figs. 2a-c.

C. freboldi Spath, 1932, p. 65, pi. 18, figs. 2a, b.

C. surense Nikitin, 1885a, p. 57, pi. 12, figs. 53-55,

text fig. 4 on p. 58.

C. shoshonense Imlay, 1948, p. 22, pi. 7, figs. 13,16,17. 

C. muelleri Imlay. 

C. tetonense Imlay. 

C. piperense Imlay.

Of these species, C. franciscus and C. piperense 

are the most compressed, C. shoshonense and C. 

tetonense are the stoutest, C. tetonense has the finest 

ribbing, C. shoshonense has the coarsest ribbing, and 

C. muelleri attains the largest size. The inner whorls 

of C. surense Nikitin are more evolute than those of 

the other species included in the group, but the deep, 

corkscrew-shaped umbilicus is quite typical. C. subpatruum Nikitin (1885a, p. 58, pi. 13, fig. 58, text 

fig. 5 on p. 59) is described as having ribbing on its 

inner whorls similar to that of C. surense Nikitin 

and may therefore belong in the same group, but its 

characteristics are more similar to those of Arcticoceras than of Cadoceras.

It is astounding that of all the seven groups of 

Cadoceras that can be recognized in the Callovian 

deposits of the Boreal sea and associated waters, 

only one occurs in the Callovian of the western interior of the United States and this particular group 

has not yet been found in the Cadoceras-rich Callovian of southwestern Alaska. The explanation is 

evidently partly stratigraphic but is probably also 

partly ecologic. In the western interior Cadoceras 

is associated with ammonites that indicate a Callovian age corresponding with the European zones of 

Macrocephalites macrocephalus, Proplanulites koenigi and Sigaloceras calloviense. In contrast, in the 

Alaskan Peninsula and the Cook Inlet regions the 

bulk of the Callovian beds are equivalent to the European Cosmoceras jason and Erymnoceras coronaturn zones of the middle Callovian. Only the basal 

fourth or fifth contain ammonites, such as Gowericeras and Gulielmiceras, representing the Proplanulites koenigi and Sigaloceras calloviense zones, and 

this basal part is not nearly so rich in ammonites as 

the overlying beds. Furthermore, in this part of

Alaska there is as yet no faunal evidence for the 

presence of beds corresponding to the Macrocephalites macrocephalus zone.

This stratigraphic explanation for the occurrence 

of groups of Cadoceras in Alaska different from 

those in the western interior is not completely satisfactory because the basal Callovian beds in Alaska 

can be correlated with the Callovian in the western 

interior on the basis of similar or identical species 

of Gowericeras, Kepplerites, Gulielmiceras, and 

Xenocephalites, and yet they contain quite different 

groups of Cadoceras. Furthermore the Alaskan beds 

contain such genera as Phylloceras, Oppelia (Oxycerites), Kheraiceras and Reineckeia that are not 

known in the western interior, and they lack nr.utiloids, which are fairly common in the Callovian of 

the western interior. These differences suggest that 

ecological factors influenced considerably the distribution of genera and subgenera of ammonites.

The Cosmoceratidae is represented abundantly by 

Gowericeras, Kepplerites (Seymourites), Cosmoceras (Gulielmiceras) and Cosmoceras (Gulielmites). 

Gowericeras is represented by six species and varieties. Of these G. subitum Imlay is the most different 

from the described European species. The genus is 

characterized by a rounded whorl section, evolute 

coiling and persistence of lateral tubercles. The venter in the western interior species is flattened in the 

young but not runcinate as in some European species. 

The species described herein are all from the middle 

part of the Rierdon formation of Montana, directly 

above the Arcticoceras beds and below the Kepplerites tychonis beds. G. costidensum Imlay occurs 

slightly lower than the other species.

Kepplerites is represented by six species, referred 

to the subgenus Seymourites. The genus may be distinguished from Gowericeras by its shorter primary 

ribs, less persistent tuberculation, more tightly coiled 

inner whorls, more persistent runcination, larger 

size, and more complicated suture. The subgenus, 

Seymourites, is retained by Spath (1932, p. 83) for 

the Boreal forms of Kepplerites because they differ 

from the typical Kepplerites in central Europe by 

their larger size, earlier loss of runcination, finer 

ribbing in the young, and by the tendency of the 

aperture to become flared. The only conspicuous 

difference is in size. Of the species from the western 

interior, one is identified with K. tychonis Ravn 

(1911, p. 490, pi. 37, fig. 1), from east Greenland and 

Alaska. The other five species occur higher stratigraphically than K. tychonis and resemble species 

from Greenland more than species from Alaska. Two 

of them, K. mclearni Imlay and K. rockymontanus
CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

Imlay, attain a larger size and have more evolute 

outer whorls than any described species of Kepplerites. In the western interior Kepplerites has been 

found mainly in the upper part of the Rierdon formation, in beds that probably correspond to the 

Sigaloceras calloviense zone of northwest Europe. 

In Alaska Kepplerites ranges through the lower twothirds of the Callovian beds that correspond to the 

Proplanulites koenigi to Cosmoceras jason zones of 

northwest Europe. Some of the species from the 

lower third show ventral flattening, as in the species 

from Montana, but those from the middle third show 

scarcely any ventral flattening, even in immature 

forms.

Gulielmiceras is represented in the western interior by three species from the Kepplerites mclearni 

beds at the top of the Rierdon formation in central 

Montana. The subgenus is characterized by persistent runcination, coarse ribbing on the body chamber, 

long lateral lappets, and presence of lateral and ventral tubercles. Most described European species of 

Gulielmiceras have a smooth or nearly smooth venter 

and a row of umbilical tubercles. All the species 

from the western interior have strong ribbing on 

the venter and lack distinct umbilical tubercles, 

although the ribs are swollen on the umbilical edge. 

Similar species are known from Alaska, East Greenland (Spath 1932, p. 96, pi. 24, figs. 3a, b, pi. 26, fig. 

5), central Russia (Nikitin, 1881a, p. 70, pi. 4, fig. 

31; Lahusen, 1883, p. 55, pi. 6, fig. 9), and England 

(Brinkman, 1929, pi. 1, fig. 1; Spath, 1932, p. 97). 

Gulielmiceras is distinguished from Kepplerites by 

its lateral lappets, greater compression, and more 

flexuous ribbing. Sigaloceras develops a smooth body 

chamber, has denser ribbing, and is generaly untuberculate. Gulielmites is distinguished by a smooth 

body whorl and an absence of lappets.

Gulielmites is represented only by three fragments 

of two species from the Kepplerites mclearni beds in 

Montana. One species is similar to C. obductum 

(Buckman) (1925, pi. 559) from England and the 

other to a form described by D'Orbigny (1845, p. 

442, pi. 36, figs. 9-15) from Russia. The presence of 

three rows of tubercles on the finely-ribbed species 

shows that it belongs in Gulielmites rather than in 

Sigaloceras.

The Perisphinctidae is represented by several species each of Procerites and Grossouvria. Although 

most of the specimens are fragments, they may be 

compared readily with species in the lower Callovian 

beds of Europe. Procerites occurs both in the Arcticoceras and Gowericeras beds in central Montana 

and in the underlying beds in association with Arctocephalites. Grossouvria has been found only in the 

beds with Gowericeras and Kepplerites.

STRATIGRAPHIC SUMMARY

Callovian ammonites have been found in thQ. western interior of the United States only in Montana, 

northern, central, and east-central Wyoming, and 

northwestern South Dakota. The Callovian beds in 

this area consist mainly of soft, calcareous gray 

shale that becomes sandier southeastward and more 

calcareous northwestward. In Montana they comprise the Rierdon formation, and in Wyoming and 

South Dakota are included in the lower part of the 

Sundance formation. Thin sandstone memberr occur 

at both the base and near the top of the sequence in 

central and eastern Wyoming and in western South 

Dakota. The basal sandstone, called the Canyon 

Springs sandstone member of the Sundance formation, fills the irregularities of an erosion surface, is 

medium-grained and partly oolitic, and greatly resembles the Nugget sandstone. It passes northwestward from the Hartville Uplift and the northern 

part of the Black Hills to the western margin of the 

Big Horn Mountains, where it grades into a sandy 

oolitic limestone that persists westward as oolitic 

limestone into Idaho and southwestern Montana. The 

upper sandstone, called the Hulett sandstone member of the Sundance formation, is fine-grained, gray, 

calcareous, thin- to thick-bedded, and is characterized by oscillation ripple marks. It thickens eastward 

at the expense of the underlying Stockade Beaver 

shale member of the Sundance formation. It extends 

westward as far as Cody in northwestern Wyoming 

and northward into southeastern Montana and most 

of North Dakota. In northwestern Wyoming west 

of Cody and Dubois and in southern Montana west 

of the Pryor Mountains it passes into dark-gray 

shale. Northwestward from southeastern Montana 

it becomes oolitic and then passes into a chalky limestone in central Montana. This limestone persists 

into Saskatchewan and marks the top of tne E ierdon 

formation over a large area. The Rierdon formation 

of Montana west of the wedge of the Huletf. sandstone member consists mainly of calcareous gray 

shale and gray limestone. Limestone predominates 

in the Little Rocky Mountains and in the subrurface 

in central Montana, but shale predominates in the 

Sweetgrass Arch area. The lower Callovian b^ds in 

Wyoming west of the wedge of the Hulett sardstone 

member consists mostly of soft gray shale but include a few thin beds of limestone. In extreme western Wyoming and southeastern Idaho the shale 

passes into a much thicker, dense limestone in the
WESTERN INTERIOR UNITED STATES

upper part of the Twin Creek limestone. This is 

overlain at the top of the Twin Creek limestone by 

ripple-marked sandstone and sandy limestone very 

similar to the Hulett sandstone member of the Sundance formation. More extended discussions of these 

^stratigraphic units may be found in other publications (Imlay, 1947,1948,1950; Imlay, Gardner, Rogers, and Hadley, 1948).

FAUNAL ZONES AND CORRELATIONS 

ARCTICOCERAS CODYENSE ZONE

The ammonite Arcticoceras characterizes the basal 

beds of the Rierdon formation in Montana and of 

the Sundance formation in Wyoming and South Dakota. As A. codyense Imlay is by far the most abundant and widespread species of the genus throughout 

this area, it is herein designated the zonal index for 

the beds containing Arcticoceras. It seems probable 

that future field work will demonstrate the existence 

of three subzones characterized from the base upward by A. rierdonense Imlay, A. loveanum Imlay, 

and A. henryi (Meek and Hayden). This is suggested by the distribution of these forms. A. rierdonense and A. n. sp. ind. have been found only in 

the western parts of Montana and Wyoming, A. loveanum and A. crassicostatum Imlay have been found 

only in eastern Wyoming, and A. henryi has been 

found in the United States only in the Black Hills 

area of easternmost Wyoming and westernmost 

South Dakota. A. codyense occurs throughout the 

entire area but is most common in central Wyoming 

and central Montana. This distribution might be explained by ecological differences, which are not obvious, or by assuming that the Callovian sea transgressed from west to east across the area in question 

and that the transgression involved sufficient time 

for several species of Arcticoceras to develop or to 

enter the sea from the polar region. The latter explanation is supported by the fact that the Callovian 

deposits of the western interior are the thickest and 

most calcareous in western Montana and southeastern Idaho and become thinner and more clastic eastward. Also, A. henryi in the Black Hills is distinctly 

at the top of the Arcticoceras beds. Failure to find 

A. henryi with A. codyense may have stratigraphic 

significance.

Arcticoceras occurs with abundant Cadoceras and 

rare Procerites in the western part of Wyoming and 

Montana. All the species of Cadoceras range into the 

overlying beds characterized by Gowericeras. Locally 

Arcticoceras is associated with many pelecypods, but 

mostly with species that range beyond the Arcticoceras beds. Gryphaea impressimarginata McLearn

attains its greatest development in the Arcticoceras 

beds in western Montana and does not range higher. 

A fairly large species of naticoid gastropod is a common associate in central Wyoming.

Arcticoceras appears to be widespread in the western interior of Canada as shown by collections at the 

University of Alberta that were examined by me at 

the request of P. S. Warren. One collection contr.ins 

single specimens of A. henryi (Meek and Hayden) 

and Gowericeras from a locality near the top of the 

Fernie shale on the Cascade River near Banff, Alberta. Another collection contains Arcticoceras and 

Arctocephalites from a locality between Bell River 

and Old Crow River near Porcupine River, Yukon 

Territory. In addition the U. S. Geological Survey 

has collections from about the same place near Porcupine River that contain Arcticoceras kochi Spath 

and Cadoceras calyx Spath.

In the Boreal region Arcticoceras has been found 

in the Petchora Basin of northern Russia (Key^erling, 1846, p. 331; Sokolov, 1912, p. 15, 49), in Fast 

Greenland (Spath, 1932, pp. 50-58, 138, 142, 145), 

and in northern Alaska on the Canning and Sadlerochit Rivers.

The age of the Arcticoceras beds of Greenland and 

Russia was determined by Spath (1932, pp. 131, T.41, 

142, 145) as lower Callovian. In Greenland Arcticoceras is associated with Cadoceras and probably vdth 

Pleurocephalites. In the Petchora Basin Arcticoceras 

is associated with Pleurocep}ialites krylowi (Milachewitch) (1879, p. 14; Sokolov^ 1912, pp. 14, 19), 

which in central Russia near Sergatsch is associated 

with Cadoceras elatmae Nikitin (Milachewitch, 1P79, 

p. 8). At several places in central Russia C. elatmae 

occurs with species of Gowericeras, Chamoussetia, 

and Macrocephalites (Nikitin, 1885b, p. 84), wHch 

are distinctly early Callovian in age. Arkell (1946, 

p. 25) shows that the zone of Cadoceras elatmae in 

Russia is equivalent to the zones of Macrocephalites 

macrocephalus and Proplanulites koenigi in northwest Europe.

The age of the Arcticoceras beds in the western 

interior of the United States is determined as basal 

Callovian because of their position at the base of a 

sequence that has furnished a succession of ammonites closely similar to that in the Proplanulites 

koenigi and Sigaloceras calloviense zones of northwest Europe. The abundance of Gowericeras immediately above the Arcticoceras beds is by itself string 

evidence for correlating the latter with the zone of 

Macrocephalites macrocephalus. The presence of 

abundant Cadoceras in the Arcticoceras beds shows 

that they are not older than the macrocephalus zone.
CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA 

Table 1.—Stratigraphic distribution of Callovian ammonite species in the western interior of the United State,-*

Rierdon formation 

of Montana

Lower part of

Sundance formation,

Wyoming and South Dakota

Genus and species

is

Kepplerite

Lilloettia sp. ind. ———•••—••—•—••——••••••—

Xenocephalites phillipsi Imlay, n. sp. — -——•••— 

X. bearpawensis Imlay, n. sp...........................•

X. cf. X. nikitini (Burckhardt)..—————•

Arcticoceras rierdonense Imlay, n. sp. ................

A. n. sp. ind.— •———•—...................................,

A. crassicostatum Imlay, n. sp.-—•——••••——

A. loveanum Imlay, n. sp. ................................

A. codyense Imlay ••—••••—••••••——•••••.......•••......

Cadoceras shoshonense Imlay .....................•••..•••

C. muelleri Imlay, n. sp.—•———•—•—••—•—— 

C. tetonense Imlay, n. sp............... ....................

C. piperense Imlay, n. sp. ..................................... ?

Kepplerites (Seymourites) mclearni Imlay —••••—

K. (Seymourites) rockymontanus Imlay, n. sp. •••••

K. (Seymourites) cf. K. rosenkrantzi Spath ——••• 

K. (Seymourites) tychonis Ravn ..........................

K. (Seymourites) landuskiensis Imlay, n. sp.——• 

K. (Seymourites) planiventralis Imlay, n. sp.••••••••

Gowericeras subitum Imlay .................................

G., subitum var. distinctum Imlay, n. var. .............

G. costihians Imlay n. sp. ...........................—•••••••

G. costimedium Imlay n. sp.— ..............................

G. costicrassum Imlay, n. sp.................................

G. costidensum Imlay, n. sp. ................................

Cosmoceras (Gulielmiceras) knechteli Imlay, n. sp 

C. (Gulielmiceras) zortmanense Imlay, n. sp.--—--. 

C. (Gulielmiceras) vigorosum Imlay, n. sp.—.— 

C. (Gulielmites) cf. C. obductum (Buckman)— 

C. (Gulielmites) cf. C. jason (Reinecke) ...........

Procerites spp. .................................................

Grossouvria warmdomensis Imlay, n. sp.............

Grossouvria spp..................................................
WESTERN INTERIOR UNITED STATES

GOWEBICEBAS COSTIDENSUM ZONE

In central and northwestern Montana from 10 to 

15 feet of the Rierdon formation directly overlying 

the basal Arcticoceras beds is characterized by a 

densely ribbed species of Gowericeras that is herein 

described as G. costidensum Imlay. This species has 

not been found with G. subitum but immediately underlies it at several localities. Associated with G. 

costidensum are numerous Cadoceras, rare specimens 

of Procerites and Grossouvria, and an abundance of 

Gryphaea nebrascensis Meek and Hayden, which 

species is exceedingly rare in the underlying beds. 

Other pelecypods are abundant locally. None of the 

species of Cadoceras is restricted to the G. costidensum zone.

Gowericeras costidensum has not been found with 

any species of Arcticoceras, although conceivably it 

may have lived at the same time as Arcticoceras 

henryi (Meek and Hayden), which has not been 

found in Montana. This possibility is suggested by 

their occurrence directly above beds containing Arcticoceras codyense. If G. costidensum and A. henryi 

coexisted, their apparent nonassociation would have 

to be explained by collecting failure or by ecological 

differences, which are not obvious. It seems more 

probable that A. henryi is slightly older than G. costidensum, especially considering that A. henryi in 

the Black Hills area is associated with A. loveanum 

Imlay, which is a common associate of A. codyense. 

The matter can be settled only by more field work.

The presence of Gowericeras is by itself excellent 

evidence for correlation with the Proplanulites koenigi zone of northwest Europe, although the genus 

occurs rarely in the Sigaloceras calloviense zone. 

Also, Procerites, typical of the Bathonian stage, is 

not recorded above the lower Callovian. The position 

of G. costidensum immediately below beds containing 

abundant Gowericeras suggests correlation with 

only the lower part of the Proplanulites koenigi zone.

GOWEBICEBAS SUBITUM ZONE

This zone is characterized by many species and individuals of Gowericeras and Cadoceras. Many of 

the latter exceed a foot in diameter. Much less common are Procerites and Grossouvria. The reported 

occurrence of Arcticoceras in this zone (Imlay, 1948, 

p. 15) was based on a misidentification of a specimen 

of Quenstedtoceras, which later collecting showed 

was derived from the basal beds of the Swift formation along with specimens of Cardioceras. Associated with the Goivericeras and Cadoceras in the G. 

subitum zone are abundant pelecypods, many large 

naticoid gastropods, and locally many brachiopods.

Gryphaea nebrascensis Meek and Hayden occurr in 

great abundance in calcareous shales and shaly limestones. Several nautiloid cephalopods have b^en 

found in this zone.

The beds characterized by Gowericeras subitum 

are correlated with the Proplanulites koenigi zone of 

northwest Europe on the basis of the abundance of 

Gowericeras, the resemblance of several species1 of 

Gowericeras to those in the Kellaways beds of Gr^at 

Britain, the highest occurrence of Procerites, and the 

stratigraphic position of the G. subitum beds, about 

90 ft. below beds containing Kepplerites, Gulielmiceras, and Gulielmites.

KEPPLEBITES TYCHONIS ZONE

The presence of finely ribbed Kepplerites tychonis 

Ravn in the middle part of the Rierdon f ormatior of 

Montana and near the base of the Chinitna siltstone 

in the Cook Inlet region of Alaska furnishes a correlation with the Kepplerites tychonis beds of East 

Greenland. These were tentatively correlated by 

Spath (1932, pp. 138, 139, 145, 146) with the Sigaloceras calloviense zone of northwest Europe, but he 

discussed the possibility that the beds may be 

younger. He was influenced by the presence of 

Gulielmiceras, which ranges from the Sigaloceras 

calloviense to the Erymnoceras coronatum zone. In 

Alaska the basal beds of the Chinitna contain s^ch 

ammonites as Gowericeras, Paracadoceras, Gulielmiceras, and Procerites, which indicate correlation 

with the Proplanulites koenigi and Sigaloceras calloviense zones. In Montana the beds containing Kepplerites tychonis are correlated with the lower part 

of the calloviense zone because of their position above 

beds containing Gowericeras and beneath beds containing Gulielmiceras and Gulielmites.

KEPPLERITES MCLEARNI ZONE

This zone has been identified in Montana at the 

top of the Rierdon formation in the Little Ro".ky 

Mountains, the Bearpaw Mountains, the East Butte 

of the Sweetgrass Hills, and the south edge of Glacier 

National Park. It is absent in the Little Belt Mountains, the Big Snowy Mountains, the Judith Mountains, most of the Sawtooth Range south of Glacier 

Park, and much of southwestern Montana, owing to 

erosion or nondeposition in late Callovian true. 

Probable equivalent beds exist in a littoral sandstone 

facies, called the Hulett sandstone member of the 

Sundance formation, throughout much of eastern 

Wyoming, western South Dakota, North Dakota, and 

eastern Montana. A similar sandstone exists at the
8 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

top of the Twin Creek limestone of southeastern 

Idaho and westernmost Wyoming.

The zone is characterized by an abundance of species and individuals of Kepplerites and Cosmoceras 

(Gulielmiceras). In addition there are a few specimens of Cosmoceras (Gulielmites), Lilloettia, Xenocephalites, and Grossouvria. Nautiloid cephalopods 

are rare. The zone also contains many pelecypods, 

including Gryphaea nebrascensis Meek and Hayden, 

and locally some brachipods. Among the peculiarities of this zone is the apparent absence of the Cadoceratinae and the resemblance of some of the species 

of Kepplerites to species of the ammonite Sigaloceras.

Formerly, Imlay (1948, p. 16) referred the ammonites herein called Lilloettia to Macrocephalites and 

those now called Xenocephalites to Kamptokephalites. The changes are based on studies of the Callovian ammonites of Alaska. The ammonites called 

Sigaloceras in the 1948 paper are now considered 

species of Kepplerites that have unusually persistent 

ventral flattening and flexuous ribbing.

The Kepplerites mcleami zone in the western interior of the United States corresponds either to the 

upper part of the Sigaloceras cattoviense zone or to 

the Cosmoceras (Gulielmites) jason zone of northwest Europe. Both Gulielmiceras and Gulielmites 

range from the upper part of the Sigaloceras calloviense zone to the Erymnoceras coronatum zones, but 

are most common above the S. calloviense zone. Kepplerites ranges from the Macrocephalites macrocephalus zone to the Sigaloceras calloviense zone in 

England and apparently into the Erymnoceras coronatum zone in northern Russia. In Alaska it occurs 

in beds which correspond to the Proplanulites koenigi, Sigaloceras calloviense, and Cosmoceras jason 

zones. Lilloettia in Alaska occurs in beds that are 

correlated with the S. calloviense and C. jason zones. 

Xenocephalites is characteristic of the Bathonian 

and lower Callovian of the Pacific Province. In 

Alaska the highest Xenocephalites is at the top of 

the local range of Gowericeras and Paracadoceras, 

which genera in Europe range as high as the S. calloviense zone but are more common in the underlying 

Callovian.

All things considered, correlation with the upper 

part of the S. calloviense zone rather than with the 

C. jason zone is favored by the first appearance of 

Gulielmiceras and Gulielmites, the abundance of 

Kepplerites, the occurrence of Xenocephalites, and 

the stratigraphic position above beds containing 

Kepplerites tychonis (which species in Alaska is associated with Gulielmiceras and is below the top of

the local range of Gowericeras and Paradeceras). 

Also, the species of Kepplerites in the K. mclearni 

zone characteristically have flattened venters This 

feature in Alaska is common only in the beds containing Gowericeras and Paracadoceras, which are considered to be older than the C. jason zone of Furope.

COMPARISONS WITH OTHER FAUNAS

Rather detailed comparisons of the Callovian ammonite assemblages of North America, the Parents 

Sea area, northwest Europe, and central Russia will 

be made by the writer in Professional Paper 249-B, 

which deals with the Callovian ammonites of the 

Alaska Peninsula and Cook Inlet regions of Alaska. 

Most of these comparisons need not be repeated. 

However, it is interesting that the Callovian ammonites of the western interior are more similar to 

those of East Greenland than to those of southwestern Alaska. This applies particularly to the genera 

Cadoceras and Kepplerites. None of the sub^enera 

of Cadoceras found in Alaska occur in the vrestern 

interior and all the species of Cadoceras in the western interior belong to a subgenus that is unkrown in 

Alaska but present in Greenland. Such ammonites 

as Gowericeras and Gulielmiceras are much more 

common in the western interior than in Alaska. Such 

genera as Paracadoceras, Pseudocadoceras, Phylloceras, and Kheraiceras are very common in Alaska 

and unknown in the western interior. Also, nautiloid 

cephalopods occur in the Callovian of the western 

interior and not in Alaska. These differences cannot 

be ascribed to land barriers because such typical 

Pacific elements as Lilloettia and Xenocephalities 

have been found in central Montana. Part of the differences may be explained by the fact that the. lowest 

Callovian in southwestern Alaska is not rich in ammonites, but most of the differences appear to be 

ecological. Thus the lithologic and faunal characteristics of the marine Callovian deposits of the western 

interior and of East Greenland indicate that the seas 

were very shallow and bordered lowlands, vhereas 

the characteristics of the Callovian deposits of southwestern Alaska indicate that fairly deep waters bordered mountainous shores.

ECOLOGIC CONSIDERATIONS 

SOURCES OF SEDIMENT

During the early Callovian the marine sediments 

deposited in the western interior of the Unitec1 States 

were derived from both east and west. East cf a line 

drawn from the Sweetgrass Arch in north-central 

Montana to the San Rafael Swell in east-central Utah
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 TABLE 2

European stages 

(Arkell, 1946)

English formations 

(After Arkell, 1933)

Northwest Europe

standard zones 

(Arkell, 1946, 1951)

Characteristic fossils in 

the western interior region

Characteristic fossils in

East Greenland, modified after

Spath (1932, 1935, 1936)

Characteristic fossils in 

the Alaska Peninsula 

and Cook Inlet regions

Alaska

Northwest side

of 

Cook Inlet

East Greenland

Jameson Land

Northern part of western interior of United States

West-central and

north-central

Montana

South-central and

southernmost

Montana

Bighorn Basin

of north-central

Wyoming

Wind River Basin 

of central 

Wyoming

Black Hills, western

South Dakota and

northeastern Wyoming

Southeastern Idaho

and 

southwestern Wyoming

Western side 

San Rafael Swell

of middle 

eastern Utah

Tertiary Lower Cretaceous Lower Cretaceous Lower Cretaceous Lower Cretaceous Lower Cretaceous Lower Cretaceous Lower Cretaceous Lower Cretaceous

Purbeck beds Titanites?, Craspedites, and 

Laugeites

Portlandian

Titanites giganteus

Kerberites okusensis

Portland beds

Glaucolithites gorei Crendonites beds

Zaraiskites albani Epipallasiceras beds

Pavlovia pallasiodes

Pavlovia rotunda Pallasiceras beds

Pectinatites pectinatus Pectinatites beds

Subplanites wheatleyensis

Subplanites sp.

Vetulonaia spp.

and 

Gyraulus veternus

Vetulonaia sp.

Kimmeridgian Kimmeridge clay Gravesia gigas

Gravesia gravesiana

Aidacostephanus pseudomutabilis

Morrison 

formation

Rasenia mutabilis

Rasenia c-ymodoce

Pictonia baylei

Hoplocardioceras decipiens

Euprionoceras kochi

Rasenia borealis

Rasenia orbignyi

Ringsteadia pseudocordata

Amoeboceras

and 

Aucella bronni

Decipia decipiens Naknek formation \

K<fch Fjaeld

formation

(Kloft I

formation)

Corallian beds

Perisphinctes cautisnigrae

Amoeboceras (Prionodoceras) 

and Ringsteadia N

Oxfordian

Perisphinctes jtlicatilis Cardioceras aff. C. zenaidae

Cardioceras cordatum Cardioceras spp.

Quenstedtoceras mariae

Cardioceras cordiforme

Quenstedtoceras collieri

Oxford clay

Quenstedtoceras lamberti

Peltoceras athleta

Erymnoceras coronatum

Callovian Kosmoceras jason

Sigaloceras cattoviense

Kepplerites mclearni

Kellaways beds

Kepplerites tychonis

Proplanulites koenigi

Gowericeras subitum

Gowericeras costidensum

Cornbrash beds

Macrocephalites macrocephalus Arcticoceras codyense

Clydoniceras discus

Clydoniceras hottandi Arctocephalites

Bathonian Great oolite

Oppelia aspidoides

Tulites subcontractus

Procerites progracilis

Zigzagiceras zigzag

Parkinsonia parkinsoni

Stemmatoceras

Stephanoceras hymphriesianum

Bajocian Inferior oolite Otoites sauzei Ckondroceras and 

Stemmatoceras

Cardioceras beds

Cardioceras distans

Cardioceras martini

Pseudocadoceras

Kepplerites, Lilloettia, 

Oxycerites

Kepplerites tychonis beds Paracadoceras, Kheraiceras, 

Xenocephalites, Gowericeras

Gulielmiceras

Chinitna siltstone

N

N

Arcticoceras beds

Arctocephalites beds

Vardeklifft

formation

(Fossil Mountain

formation)

Cranocephalites beds Cranocephalites

Sphaeroceras, Oxycerites

Chondroceras

Normannites, Teloceras \

Stemmatoceras

Sonninia sowerbyi Emileia

Ludwigia murchisonae

Tmetoceras

Lioceras opalinum a, ^

Tuxedni formation

Lnftoceras jurense Pseudolioceras

Hildoceras bifrons Dactylioceras

Toarcian Upper Lias Pseudogrammoceras

Harpoceras serpentinum

Dactylioceras tenuicostatum Neills Cliff formation

Middle Lias

Paltopleuroceras spinatum

Present

Amaltheus margaritatus

Pliensbachian Prodactylioceras davoei

Amaltheusf?), Deroceras 

and Xipheroceras

\

\

Tragophylloceras ibex Beaniceras

Uptonia jamesoni Uptonia jamesoni

Probably

represented

by tuffs,

lavas, and

agglomerates

Echioceras raricostatum

Oxynoticeras oxynotum

Lower Lias Asteroceras obtusum

Sinemurian

Arietites turneri Absent

Coroniceras and 

Amioceras

Arnioceras semicostatum

Coroniceras bucklandi

Scamnoceras angulatum

Hettangian Wahneroceras (?)

\

\

Swift formation Swift formation

Rierdon formation

Rierdon formation

Psiloceras planorbis Plant beds Cape Stewart 

formation (part)

Marine

yellowish

siltstone

and

silty

sandstone

Dark gray 

shale and 

limestone 

with basal 

gray sandstone

Red shale

and

siltstone 

Some gypsum

Marine shale 

and limestone 

Locally pebbly

Massive gypsum

and 

red beds

Morrison 

formation

Morrison 

formation

Morrison 

formation

Glauconitic 

sandstone 

and shale

Conglomerate 

at base 

locally

Glauconitic 

sandstone 

and shale

Conglomerate 

at base 

locally

Gray sandstone

Gray shale and 

thin limestone

Oolitic limestone 

and sandstone

Red and gray

shale; thin beds

of gypsum; some

limestone that

is locally

pebbly

Gray sandstone

Gray shale and 

thin limestone

Sandy oolitic 

limestone

Red shale

chert rosettes

common, some

gypsum

Dolomite, 

limestone and 

shale, locally 

pebbly at base

Morrison 

formation

Redwater

shale 

member

Stump sandstone

Lak member Preuss sandstone

Hulett ss. member

Stockade Beaver 

shale member

Canyon Springs 

sandstone member

Massive gypsum,

some limestone,

and red beds

Gypsum Spring 

formation

Gypsum Spring 

formation

Nugget sandstone

Sandy limestone

Shaly limestone

Limestone, 

partly oolitic

Red beds

and 

yellowish

silty 

limestone

Medium gray 

shaly limestone

Medium-bedded 

limestone

Red beds, 

limestone,

partly 

brecciated

Summerville 

formation

Curtis 

formation

Entrada sandstone

Carmel formation

Nugget sandstone. Navajo sandstone

Ka\enta 

formation

Wingate 

sandstone

Upper Triassic Upper Triassir

Pennsylvanian to 

Mississippian

Triassic to 

Pennsylvanian

Triassic Triassic Triassir Triassic Triassic

CORRELATION OF JURASSIC FORMATIONS IN PARTS OF THE WESTERN INTERIOR OF THE UNITED STATES 209238 O - 52 (Face p. 8)
WESTERN INTERIOR UNITED STATES

the dominant source of sand was to the east or southeast. West of this line the dominant source of sand 

was to the west. Apparently most of the clay came 

from the east, as nearly all shaly units become more 

calcareous westward. However, in the Sawtooth 

Range in western Montana the lower Callovian sediments become shalier northward toward the Glacier 

Park area and equivalent beds in the front ranges 

of Alberta are essentially all shale. Locally some islands in central Montana and north-central Wyoming furnished sand and pebbles at the beginning of 

the Callovian transgression.

CONDITIONS OF DEPOSITION

CONGLOMERATE

Small pebbles of limestone occur at the base of the 

Rierdon formation on the East Butte of the Sweetgrass Hills in north-central Montana. It is probable 

that the pebbles were derived from outcrops of Mississippian limestone exposed on the Sweetgrass Arch 

at the beginning of the Callovian.

Pebbles of dark-gray chert, quartz, and quartzite 

characterize the base of the Sundance formation in 

areas adjoining the Big Horn Mountains and the 

Black Hills. Most of the pebbles are less than an inch 

in diameter, and consist of irregularly pitted, highly 

polished, dark-gray chert identical with chert pebbles in the Middle Jurassic beds of north-central 

Wyoming. Some pieces of the quartz and quartzite 

exceed three inches in diameter. In both areas the 

pebbles occur as thin layers in soft, gray, calcareous 

shale, or sandstone, or are disseminated through several feet of gray shale. On the west side of the Big 

Horn Mountains near Tensleep, a basal conglomerate 

contains some pebbles and cobbles of dolomitic limestone and of rounded siliceous rosettes identical with 

material in the underlying Middle Jurassic beds. 

Judging from the distribution of these pebbly beds, 

the sources of the pebbles were landmasses in the Big 

Horn area and the Black Hills area. The identity of 

the pitted and polished chert pebbles with some in 

the Middle Jurassic of north-central Wyoming suggests that they came from the same source and were 

residual pebbles, swept basinward by an advancing 

sea. The presence of siliceous rosettes in the pebbly 

beds indicates that the Middle Jurassic beds, adjoining the Big Horn Mountain area, underwent erosion 

before the Callovian transgression.

A third occurrence of pebbles in the Callovian is 

in the basal coarse-grained sandstone of the Sundance formation in the Bison Basin area in northwestern Sweetwater County, south-central Wyoming (Love and others 1945). The pebbles evidently

were derived from the south or southeast, as the 

area is near the southeastern margin of marine Callovian deposits.

SAND

Lower Callovian sandy material occurs mainly in 

the eastern part of the western interior seaway 

north of Colorado. A thin mantle of sand, called the 

Canyon Springs sandstone member of the Sundance 

formation, lies at the base of the Callovian in southern and eastern Wyoming and parts of adjoining 

states (Imlay, 1947, pp. 232, 233, 247-251; Love, 

1948, p. 103), fills in the irregularities of an erosion 

surface, and locally attains 60 ft. in thickness. 

It consists mostly of medium to coarse grains, of 

which many are well-rounded and frosted, but contains some oolites and beds of oolitic limestone and 

passes northward in southern Montana and we^tward in central Wyoming into oolitic limestone and 

shale. It greatly resembles the Nugget sandstone, 

from which it was probably derived either early in 

the Callovian transgression or immediately preceding it. In the latter case, this basal sand may hs.ve 

been deposited first by winds or streams and later 

reworked by the sea. The latter possibility is suggested by the abrupt contact with the overlying 

Stockade Beaver shale member in most places.

Equivalent oolitic limestones at the base of the 

Callovian in southeastern Idaho contain some sand 

and become sandier westward. The degree of sandiness is comparable to that of the Canyon Sprirgs 

sandstone member of the Sundance formation at the 

south end of the Big Horn Mountains, where the 

source area of the sand was at least 150 miles to the 

southeast.

Another mantle of sand, called the Hulett sandstone member of the Sundance formation, occurs at 

the top of the lower Callovian sequence and is a little 

thicker and more widespread in Wyoming and eastern Montana than the basal sand. It ranges in thickness from a few feet to 12 ft. and thickens to the 

southeast. It is grayish, thin- to thick-bedded, finegrained, calcareous, glauconitic, ripple-marked, and 

fairly hard (Imlay, 1947, pp. 255-257). It passes 

westward in northwestern Wyoming into calcareous 

shale and northward in central Montana into oolitic 

to chalky limestone. It differs from the basal sand 

by its smaller grain size, presence of glauconite, and 

numerous oscillation ripple marks. It must hrve 

been derived from rocks older than the Nugget sandstone, because that formation was nearly covered by 

the end of the lower Callovian. It must have been 

deposited in fairly shallow water, as indicated by 

ripple marks. The absence of current ripple marks
10 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

and scarcity of cross bedding suggests deposition 

some distance from shore. Bottom conditions were 

evidently unfavorable for life or for its preservation, 

as fossils are rare in the sandstone and consist generally of free-swimming or free-floating forms, such 

as Pentacrinus and Camptonectes. The latter are 

generally fragmentary. Deposition of the Hulett 

sandstone member in the littoral zone is indicated by 

the abundance of ripple marks, the scarcity and 

fragmentary conditions of the fossils within it, and 

superposition directly on the Stockade Beaver shale 

member, whose shallow-water origin is attested by 

an abundance of Lingulas. Judging by the extent of 

the Hulett sandstone member, sand flats were widely 

exposed during low tide along the eastern margin of 

the lower Callovian sea.

A similar glauconitic sand was deposited in southeastern Idaho near the end of the early Callovian. 

It was derived from the west, as it passes eastward 

into sandy to silty, ripple-marked limestone at the 

top of the Twin Creek limestone. Evidently the sea 

became very shallow at the end of the early Callovian just prior to the deposition of the red Preuss 

sand.

MARINE snVTSTONE AND SHALE

Lower Callovian calcareous gray shale and siltstone, known as the Stockade Beaver shale member 

of the Sundance formation in Wyoming and the 

Dakotas, grade northwestward from a featheredge 

in southwestern Wyoming and western South Dakota into a shale and limestone sequence (Rierdon 

formation), locally more than 200 ft. thick in northwestern Wyoming and western Montana. To the 

southeast, the shale and siltstone of the Stockade 

Beaver shale member are underlain and overlain by 

sandstones of early Callovian age. Silt decreases 

northwestward as the adjoining sandstones thin, and 

does not occur much farther west than the western 

margin of the overlying Hulett sandstone member 

of the Sundance formation, except locally near Belt 

Island in west-central Montana. In westernmost 

Wyoming the shale and limestone sequence passes 

into as much as 1500 ft. of shaly limestone in the 

upper part of the Twin Creek limestone. The Stockade Beaver shale member contains many ostracodes 

and some Foraminifera, and Lingulas. Pelecypods, 

and ammonites have been found associated with calcareous concretions near its base. The Rierdon formation likewise has many ostracodes, but also many 

Foraminifera, mollusks, and brachiopods. It is especially characterized by an abundance of Gryphaea, 

which is uncommon in the more silty facies, such as 

the type Stockade Beaver member, or in the equivalent limestone facies in the upper part of tl Q. Twin 

Creek limestone. Very shallow water deposition is 

indicated for the Rierdon formation by th« abundance of Gryphaea and by numerous pelecypods, and 

for the Stockade Beaver shale member by the presence of linguloid brachiopods (Twenhofel anc1 others, 

1932, p. 175; Allan, 1936, pp. 383-385).

IJMESTONE

Most of the limestone of Callovian age was formed 

in the western part of the Jurassic seaway and the 

main area of deposition was in southeastern Idaho 

and north-central Utah. This limestone, representing the upper two-thirds of the Twin Creek limestone, is mostly shaly to thin-bedded, dense to porcelaneous, and poorly fossiliferous. The most abundant fossils are free-swimming or free-floating forms 

such as belemnites, crinoids, and Camptonectes. Bottom-dwelling pelecypods, such as Astarte, Trigonia, 

and Pinna occur in a few beds. Gryphaea occurs only 

in the basal beds and along the eastern mr.rgin of 

the facies, where it is becoming shalier. Such features as fine texture and scarcity of organic matter 

and fossils suggest a chemical origin for ths limestone.

Most of the lower Callovian limestone in 3\Tontana 

and northwestern Wyoming consists of thin beds in 

a dominantly shale facies, except for an area in central Montana centering around the Little Rocky 

Mountains and lying immediately east of Belt Island. 

These limestones generally contain many rrollusks, 

including Gryphaea and Ostrea, that prove deposition in fairly shallow water.

OTHER TYPES OF SEDIMENT

Bedded gypsum of probable early Callovian age 

occurs in the upper part of the Carmel formation in 

Utah associated with gray, greenish, and reddish 

shale and with limestone. The gypsiferous bods pass 

eastward and southward into thinner red shale and 

sandstone and westward into thicker limestone and 

gray shale. The thin red beds cover large r.reas in 

eastern Utah and adjoining parts of Colorado, New 

Mexico, and Arizona. They are considered to be 

marginal deposits formed at the southern end of a 

seaway. Their early Callovian age is based entirely 

on stratigraphic position and on lateral trying of 

units northwestward into the Twin Creek Milestone.

Siliceous material in lower Callovian beds has been 

observed by the writer only near the top of the 

Hulett sandstone member of the Sundance formation 

along the southern base of the Pryor Mountains in 

southern Montana. The material consists of light 

gray rosettes or irregular rounded masses, some as
WESTERN INTERIOR UNITED STATES 11

much as a foot in diameter. One of the most accessible localities to observe these is along Gypsum 

Creek in the SE 14 of Sec. 33, T. 9 S., R. 27 E., Carbon County. Similar siliceous rosettes are widespread in Wyoming in the upper part of the Middle 

Jurassic. In both cases the siliceous masses lie in 

the upper parts of lithologic units whose upper surfaces are cut by minor disconformities. This relationship suggests that the presence of these siliceous 

masses may be related to times of erosion.

Minor amounts of glauconite have been found in 

the Hulett sandstone member of the Sundance formation in eastern Wyoming and western South Dakota 

and at the top of the Rierdon formation in northwestern Montana. Some of the ripple-marked sandy 

beds at the top of the Twin Creek limestone in southeastern Idaho contain an abundance of glauconite. 

The presence of glauconite appears to be related to 

shallowing of the sea during its withdrawal north of 

Montana just before deposition of red beds.

Oolites were formed widely in the western interior 

at the begining and end of the Callovian marine 

transgression. They are interpreted as indicating 

very shallow water and saturation of the water with 

calcium carbonate.

AMMONITE DISTRIBUTION AND ASSOCIATIONS

The writer has discussed the faunal assemblages 

in the Callovian sea of the western interior region 

and presented evidence that the limestones and 

shales were deposited in the upper part of the neritic 

zone and that the sandstone members were probably 

deposited partly in the littoral zone (Imlay, 1950, 

pp. 94-99). The littoral character is suggested by 

the abundance of ripple marks, a scarcity of benthonic mollusks, the fragmentary character of specimens of Camptonectes, and the presence of oysters 

and Mytilus locally. General deepening of the sea 

westward across Wyoming and Montana, except 

where islands occurred, is indicated by an increase in 

the number of Gryphaeas and foraminifers and a 

decrease in the number of ostracodes and Lingulas.

The ammonites are most abundant in thin limestones that are interbedded with calcareous shale 

and contain many benthonic pelecypods. Many of 

these thin limestones are slightly silty or sandy. 

Some of the associated beds may be oolitic. Ammonites are extremely rare in thick-bedded or ripplemarked sandstones, especially in beds that contain 

Ostrea or Mytilus. The absence of ammonites in the 

dense, shaly limestone forming the upper two-thirds 

of the Twin Creek limestone is possibly related to 

the scarcity of pelecypods on which they fed and

may perhaps be explained by unfavorable bottom 

conditions induced by rapid subsidence and equally 

rapid sedimentation and perhaps, also, by abnormally 

high salinity of the sea water. Evidently the ammonites favored those parts of the sea beyond the littoral zone in which there was an abundance of bottom-dwelling mollusks, fairly hard bottoms, and 

slow deposition of sediments. These parts were most 

widespread in the areas now included in northwestern Wyoming and central and northwestern 

Montana.

Certain features in the distribution and associations of the ammonite genera in the western interior 

sea during early Callovian time indicate that their 

presence was controlled or influenced greatly by 

depth of water and perhaps by temperature. For 

example, Arcticoceras is the most widespread of all 

the genera, specimens being found in eastern Wyoming in oolitic sandstones, in central Wyoming in 

oolitic limestones, and in western Montana in thinbedded limestones interbedded with shales. It is associated with ammonites such as Cadoceras, Procerites, and Grossouvria only in'sequences that contain some shales and were presumably formed in 

deeper water than the sequences that are domins ntly 

oolitic. As another example, specimens of the g?.nus 

Gowericeras have been found in central and western 

Montana in thin-bedded limestones that may be 

slightly silty, sandy, or oolitic. It is associated with 

many specimens of Cadoceras and some of Procerites, Grossouvria, and nautiloid cephalopods. It has 

not been found with Kepplerites except possible at 

one locality on Rierdon Gulch in western Montana 

where two fragments of a single specimen of Kepplerites (U. S. G. S. Mes. loc. 22650) were obtained 

as float about 40 ft. apart on a steep slope near the 

top of the Arcticoceras beds. It seems likely that this 

specimen was derived from the Gowericeras subitum zone which forms the top of the Rierdon formation at Rierdon Gulch. The scarcity of Kepplerites 

in the Gowericeras beds is striking because in the 

overlying beds of the Kepplerites tychonis zone the 

genera Cadoceras and Kepplerites are common^ associated. Also, in Alaska and in the Boreal region 

Gowericeras, Kepplerites, and Cadoceras are associated.

The most peculiar cephalopod association is that 

represented by the Kepplerites mclearni zone, which 

includes Kepplerites, Cosmoceras (Gulielmiceras), 

C. (Gulielmites), Grossouvria, Xenocephalites, Lilloettia, and a few nautiloids, but not a single Cadoceras. In contrast, equivalent beds in Alaska and the 

Boreal region contain numerous Cadoceras. Peculiar
12 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

also is the absence from the western interior of such 

ammonites as Paracadoceras, Pseudoeadoceras, Phylloceras, and Kheraiceras (which are so abundant in 

Alaska), and the occurrence of only one of the several subgenera of Cadoceras present in the Boreal 

region. That the seas were connected is shown by 

the presence of such typical Pacific elements as 

Xenocephalites and Lilloettia as far south as Montana. It is also shown by the occurrence of Kepplerites tychonis in Alaska, Montana, and the Poreal 

region.

Judging from these occurrences, it seems probable 

that Arcticoceras could live in shallow waters of 

various depths nearly to the strand line, that Gowericeras, Cadoceras, Procerites, and Grossouvria could 

not live in as shallow waters as Arcticoceras, and 

that Kepplerites preferred still deeper waterr than 

Gowericeras. The absence of Cadoceras in the K&p-

_ CANADA

STATES

! N o R T ,H D A K 6

i

P;5Bf^lPllii^iw i ^*ss^ <*s

FIGURE 1.—Index map showing Callovian localities in the western interior United States.
Table 3.—Geographic distribution of the Callovian Ammonites of the western interior of the United States

CosmoceTag (Gulidmiceras) knechteti Imlay, n. sp. . .

Montana Wyoming

Rierdon 

formation

3

X

X

34

X

„

S

X

00

X

9

S

g

S

M

i i 1

to1 i i i

§CD 

r» i i 1

S

S i

-

o 

o

•

I

5O

1

31 

O>

1

r>

Jo i

a 

=

-

1

X

X

i

X

X

39

X

X

i

-

So

X

X

i

X

So

-

»

1

X

X

o i

v

§

v

<M 

M 5

o

i i

QO

3

»

OS i i

.

CD

tX

0

I

X 

X

X

X

X

S

01

X 

X

X

X

X

o

X 

X

X

X

X

S

X 

X

X

X

i

_

X

—

to

:D

-

-

X

-

S

X 

X

X

-

OS

X

X

OK

X 

X

X

2

94

ra

X

a

XI 

OK

X

2

as N

X

o 

so

X

1

X

us a

00 e»

C '

? >

2 &

!§

c -

South 

Dakota

Sundance 

formation

QO

2 S

X

i

X

2

1

94

X

S

1

<M

M

X

a

CD 

M

X 

X

i

X 

X

<M

X

ei <M

111

y - x

-XX

M

CD 

1

X 

X

WESTERN INTERIOR UNITED STATES 13

20*

140

150°

I60°fe

170

160

120' 110° 100" 90° 

FIGURE) 2.—Lower Callovian paleogeography of the Boreal region.

80°
14 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

plerites mclearni zone may be due to a shallowing 

and possible warming of the early Callovian sea during its withdrawal northward or to the extinction of 

the single subgenus that managed to migrate into 

the western interior from the Boreal region. The 

absence of many ammonite genera in the Callovian 

beds of the western interior that are present in the 

Callovian of southwestern Alaska is likely to be related to greater shallowness and warmth of the 

western interior sea. Some of these puzzling faunal 

relationships will probably be solved when the Callovian faunas of the western interior of Canada have 

been more fully studied.

GEOGRAPHIC DISTRIBUTION

The occurrence by area and locality of the 3£ species described herein is shown in the table facing 

p. 12. The general position of each locality is shown 

on figure 1. Detailed descriptions of the individual 

localities are given in the following list to avoid 

repetition in the text:

Localities at which ammonites were collected from Cattovian beds in the 'western interior of the United States

No. on

fig. 1

1 _

1 ...

2 _

3—

3.__

3 _

3—

3—

3 _

3 _

Geological Survey Mesozoic

locality

7124

19194

20390

18718

19397

19590

19591

19593

19594

19596

Collector, year of collection, description of locality, and stratigraphic

assignment

M. R. Campbell and T. W. Stanton,

1911. On Great Northern Ry, from

^ to M mile west of Skyland, in the

NW* of sec. 8, T. 29 N., R. 14 W.,

Flathead County, Mont. Rierdon

formation, Kepplerites mclearni zone.

R. W. Imlay and Hal C. Yingling, Jr.,

1944. One mile east of Autumn

Creek on north side of U. S. Highway

2, in the NWJ of sec. 9, T. 29 N., R.

14 W., Flathead County, Mont.

Rierdon formation, Kepplerites tychonis zone.

W. A. Cobban and R. W. Imlay, 1946.

North slope of Sawmill Creek, ^ mile

above mouth, in sec. 3, T. 29 N., R.

11 W., Pondera County, Mont. About

19 ft. below top of Rierdon formation,

Kepplerites tychonis zone.

Josiah Bridge and C. F. Deiss, 1941.

Lonesome ridge, west side of sec. 34,

T. 25 N., R. 9 W., Teton County,

Mont. Rierdon formation.

W. A. Cobban, 1944. Head of Rierdon

Gulch, sec. 23, T. 24 N., R. 9 W.,

Teton County, Mont. Twenty feet

above base of Rierdon formation,

Arcticoceras codyense zone.

R. W. Imlay and William Saalfrank,

1945. Near line of sees. 33 and 34,

T. 25 N., R. 9 W., Sawtooth Range,

Teton County, Mont. About 36 ft.

below top of Rierdon formation,

Gowericeras beds.

William Saalfrank, 1945. Near line

of sees. 33 and 34, T. 25 N., R. 9 W.,

Teton County, Mont. Float from

Rierdon formation.

'William Saalfrank, 1945. Near line

of sees. 33 and 34, T. 25 N., R. 9 W.,

Teton County, Mont. About 55 ft.

above base of Rierdon formation,

Arcticoceras codyense zone.

R. W. Imlay and William Saalfrank,

1945. Near line of sees. 33 and 34,

T. 25 N., R. 9 W., Teton County,

Mont. Ten ft. above base of Rierdon

formation, Arcticoceras codyense zone.

R. W. Imlay and William Saalfrank,

1945. Near line of sees. 33 and 34,

T. 25 N., R. 9 W., Teton County,

Mont. About 16 ft. below top of

Rierdon formation. Gowericeras beds.

No. on

fig. 1

3—

3—

3—

3...

3—

3—

3 ...

3 _

3—

4__.

Geological Survey Mesozoic

locality

19597

19599

19600

19602

19603

19605

19607

20365

22651

18610

Collector, year of collection, description of locality, and stratigraphic

assignment

R. W. Imlay and William Saalfrank,

1945. Near line of sees. 33 and 34,

T. 25 N., R. W., Teton County,

Mont. About 40 ft. above b«se of

Rierdon formation, Arciir.oce.ras

codyense zone.

R. W. Imlay, 1945. Line of sees. 33

and 34, T. 25 N., R. 9 W., Teton

County, Mont. From upper third

of Rierdon formation.

R. W. Imlay and William Saalfrank,

1945. Near line of sees. 33 and 34,

T. 25 N., R. 9 W., Teton County,

Mont. From lower 40 ft. of R^rdon

formation, Arcticoceras codyense zone.

R. W. Imlay and William Saalfrank,

1945. Near line of sees. 33 and 34,

T. 25 N., R. 9 W., Teton County,

Mont. About 8 ft. below top of

Rierdon formation, Gowericeras subitum zone.

A. G. Alpha, 1945. Near line of sees.

33 and 34, T. 25 N., R. 9 W., Teton

County, Mont. About 20 ft. below

top of Rierdon formation, Gowericeras

beds.

William Saalfrank, 1945. Near line of

sees. 33 and 34, T. 25 N., R. 9 W.,

Teton County, Mont. About 30 ft.

above base of Rierdon fornration,

Arcticoceras codyense zone.

R. W. Imlay and William Saalfrank,

1945. Sec. 21, T. 25 N., R. 9 W.,

Teton County, Mont. From 25 to

30 ft. below top of Rierdon fornration,

Gowericeras beds.

William Cobban and R. W. Imlay, 1946.

Near line of sees. 33 and 34, T. 25 N.,

R. 9 W., Teton County, Mont. Upper

15 ft. of Rierdon fornration,

Gowericeras subitum zone.

R. W. Imlay, 1951. South central part

of sec. 23, T. 24 N., R. 9 W., Teton

County, Mont. Rierdon fornration,

about 38 ft. below top.

C. F. Deiss and R. M. Garrells, 1940.

Ridge east of head of East Fork of

Lange Creek, in the SE|, sec. 35, T.

21 N., R. 9 W., Lewis and Clark

County, Mont. Rierdon fornration,

Arcticoceras codyense zone.
WESTERN INTERIOR UNITED STATES 15

Localities at which ammonites were collected from Callovian beds in the western interior of the United States—Continued

No. on 

fig. 1

5—

6--_

6—

6—

7—

7—

8—

8—

8—

8—

8—

8— _

8 _

Geological Survey Mesozoic

locality

20391

11973

19196

. 21467

18758

19202

10729

10730

10732

18726

18735

18737

18738

Collector, year of collection, description of locality, and stratigraphic

assignment

Bert Brush, 1946. Northern Ordnance

Inc. Holtz No. 1 well, sec. 23, T. 37

N., R. 2 W., Toole County, Mont.

Top of Rierdon formation, Kepplerites

tychonis zone.

A. J. Collier, 1923. East side of East

Butte of Sweetgrass Hills, sec. 28, T.

36 N., R. 5 E., Liberty County, Mont.

Rierdon formation, Kepplerites

mclearni zone.

C. E. Erdmann, 1930. North side of

East Butte of Sweetgrass Hills, SEJ

of sec. 8, T. 36 N., R. 5 E., Liberty

County, Mont. Upper 33 ft. of

Rierdon formation, Kepplerites

mclearni zone.

A. R. Loeblich and R. W. Imlay, 1948.

North side of East Butte of Sweetgrass Hills, SEi of sec. 8, T. 36 N.,

R. 5 E., Liberty County, Mont.

About 20 ft. below top of Rierdon

formation, Kepplerites mclearni zone.

M. M. Knechtel, S. W. Hobbs, and J.

B. Reeside,, Jr. 1938. Section Creek,

sec. 1, T, 28 N., R. 20 E., east end of

Bearpaw Mountains, Blaine County,

Mont. Rierdon formation, Kepplerites mclearni zone.

R. W. Imlay and Hal C. Yingling, Jr.,

1944. Sec. 1, T. 28 N., R. 20 E., at

east end of Bearpaw Mountains,

Blaine County, Mont. From 25 to

35 ft. above base of Rierdon formation, Gowericeras subitum zone.

A. J. Collier, T. W. Stanton, 1921.

One mile a little north of east of the

St. Paul Mission, Little Rocky

Mountains, Blame County, Mont.

Rierdon formation, Kepplerites

mclearni zone.

T. W. Stanton and L. Horton, 1921.

One mile S. 65° W. of Landusky,

Little Rocky Mountains, Phillips

County, Mont. Rierdon formation,

Kepplerites mclearni zone.

T. W. Stanton, 1921. East slope of

mountain south of Zortman, Phillips

County, Mont Rierdon formation,

Kepplerites mclearni zone.

J. B. Reeside, Jr., and S. W. Hobbs,

1938. In the W£ of sec. 7, T. 24 N.,

R. 25 E., on Morrison Butte, Little

Rocky Mountains, Phillips County,

Mont. Rierdon formation, Kepplerites mclearni zone.

M. M. Knechtel, S. W. Hobbs, and J. B.

Reeside, Jr., 1938. In the SEJ of

sec. 12, T. 25 N., R. 25 E., Little

Rocky Mountains, Phillips County,

Mont. Rierdon formation, Kepplerites mclearni zone.

M. M. Knechtel, S. W. Hobbs, and J. B.

Reeside, Jr., 1938. In the NW£ of

sec. 3, T. 25 N., R. 26 E., Little Rocky

Mountains, Phillips County, Mont.

Rierdon formation, Kepplerites mclearni zone.

M. M. Knechtel, S. W. Hobbs, and J. B.

Reeside, Jr., 1938. In the SE| of

sec. 15, T. 25 N., R. 25 E., Little

Rocky Mountains, Phillips County,

Mont. Rierdon formation, Kepplerites mclearni zone.

No. on 

fig. 1

8_._

8— _

8___

8—

8___

8—

8—

8—

8...

8—

8_._

8...

Geological Survey Mesozoic

locality

18742

18750

18753

18754

19203

19211

19212

19572

19576

19583

19584

19585

Collector, year of collection, description of locality, and stratigraphic

assignment

M. M. Knechtel, S. W. Hobbs, and J. B.

Reeside, Jr., 1938. In the SEi of

sec. 32, T. 24 N., R. 24 E. Little Ro?ky

Mountains, Phillips County, Mont.

Rierdon formation, Kepplerites

mclearni zone.

M. M. Knechtel, S. W. Hobbs, 1P38.

Middle of S| of sec. 5, T. 24 N., R.

26 E., Little Rocky Mountains,

Phillips County, Mont. Rierdon formation, Kepplerites mclearni zone.

M. M. Knechtel, S. W. Hobbs, and J.

B. Reeside, Jr., 1938. Small dome

just south of Camp Creek dome, in

the S| of sec. 5, T. 24 N., R. 26 E.,

Phillips County, Mont. Rierdon formation, lower 10 ft.

M. M. Knechtel, S. W. Hobbs, and J. B.

Reeside, Jr., 1938. East of Jim

Brown's Creek, in the NEj of sec.

T. 26 N., R. 24 E.. Little Rocky

Mountains, Blaine County, Mont.

Rierdon formation, Kepplerites

mclearni zone.

R. W. Imlay and Hal C. Yingling, Jr.,

1944, Sec. 7, T. 25 N., R. 26 E.,

Chalk Butte dome, Phillips Courty,

Mont. Upper 10 ft. of Rierdon formation, Kepplerites mclearni zone.

R. W. Imlay and Hal C. Yingling, Jr.,

1944. Sec. 32, T. 25 N., R. 24 E.,

one mile southwest of Landusky,

Phillips County, Mont. Upper 10

ft. of Rierdon formation, Kepplerites

mclearni zone.

R. W. Imlay and Hal C. Yingling, Jr.,

1944. Sec. 7, T. 25 N., R. 26 E.,

Chalk Butte dome, Phillips County,

Mont. From 11 ft. of nodular limestone which is 13 ft. above base of

Rierdon formation, Gowericeras

costidensum zone.

William Saalfrank, 1945. Sec 7, T.

25 N., R. 26 E., Chalk Butte dome,

Phillips County, Mont. Upper 10

ft. of Rierdon formation, Kepplerites

mclearni zone.

R. W. Imlay and William Saalfrank,

1945. NW corner of sec. 6, T. 24 N.,

R. 26 E., Little Rocky Mountains,

Phillips County, Mont. Rierdon

formation, lower 10 ft. just above a

bed of yellow sandy limestone,

Gowericeras costidensum zone.

R. W. Imlay and William Saalfrank,

1945. NW corner of sec. 6, T. 24 N.,

R. 26 E., South side of Camp Creek

dome, Phillips County, Mont. Upoer

10 ft, of Rierdon formation, kepp^rites mclearni zone.

R. W. Imlay and William Saalfrank,

1945. Sec. 3, T. 25 N., R. 26 E., "

east side of Matador dome, Phillips

County, Mont. Upper 10 ft. of

Rierdon formation, Kepplerites

mclearni zone.

R. W. Imlay and William Saalfrank,

1945. Sec. 32, T. 26 N., R. 26 E.,

northeast end of Little Warm dome,

Phillips County, Mont. From 10 to

15 ft. above base of Rierdon fomation, Gowericeras costidensum zone.
16 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

Localities at which ammonites were collected from Callovian beds in the western interior of the United States—Continued

No. on

fig. 1

8—

8—

8—

9—

10.._

10_._

10...

10 _

10...

10—

11 —

11 _

11 _

Geological Survey Mesozoic

locality

19587

19588

19589

19176

19169

19179

19180

19181

19213

19616

19170

19175

19182

Collector, year of collection, description of locality, and stratigraphic

assignment

R. W. Imlay and William Saalfrank,

1945. Sec. 3, T. 25 N., R. 26 E.,

east side of Matador dome, Phillips

County, Mont. Rierdon formation,

about 20 ft. above base.

R. W. Imlay, 1945. Sec. 7, T. 25 N., 

R. 26 E., Chalk Butte Dome, Phillips

County, Mont. From base of nodular limestone, 13 ft. above base of

Rierdon formation, Arcticoceras codyense zone.

R. W. Imlay and William Saalfrank,

1945. Sec. 32, T. 26 N., R. 26 E.,

northeast end of Little Warm dome,

Phillips County, Mont. From 1 ft.

of sandy limestone from 3 to 4 ft.

above base of Rierdon formation,

Arcticoceras codyense zone.

R. W. Imlay and Hal C. Tingling, Jr.,

1944. Directly east of Giltedge

Mine, in the SWJ of sec. 17, T. 16 N.,

R. 20 E., Fergus County, Mont.

Rierdon formation, lower 30 ft.

R. W. Imlay, Hal C. Yingling, Jr., and

Roland Brown, 1944. Sec. 15, T. 14

N., R. 20 E., Fergus County, Mont.

Rierdon formation, from 15 to 30 ft.

above base, Oowericeras subitum zone.

R. W. Imlay and Hal C. Yingling, Jr.,

1944. In the SE| of sec. 17, T. 14

N., R. 20 E., Fergus County, Mont.

From 10 to 30 ft. above base of

Rierdon formation, Gowericeras subitum zone.

J. B. Reeside, Jr., Oscar Mueller, and

R. W. Imlay, 1944. In the SEJ of

sec. 9 and NEfc of sec. 16, T. 14 N.,

R. 20 E., Fergus County, Mont.

From 10 to 30 ft. above base of

Rierdon formation, Gowericeras subitum beds.

J. B. Reeside, Jr., Oscar Mueller, and

R. W. Imlay> 1944. North side of

road in north central part of sec. 14,

T. 14 N., R. 20 E., Fergus County,

Mont. From lower 25 ft. of Rierdon

formation, Gowericeras subitum zone.

Oscar Mueller, 1946. Sec. 16, T. 14

N., R. 20 E., Fergus County, Mont.

Rierdon formation, Gowericeras subitum zone.

R. W. Imlay and William Saalfrank,

1945. Near middle of sec. 12, T. 14

N., R. 19 E., Fergus County, Mont.

Lower 30 ft. of Rierdon formation,

Gowericeras subitum zone.

R. W. Imlay and Hal C. Yingling, Jr.,

1944. Near center of sec. 15, T. 14

N., R. 21 E., Fergus County, Mont.

From lower 15 ft. of Rierdon formation, Gowericeras costidensum zone.

R. W. Imlay and Hal C. Yingling, Jr.,

1944. East side of road in north

central part of sec. 14, T. 14 N., R.

21 E., Fergus County, Mont. From

lower 22 ft. of Rierdon formation,

Gowericeras subitum zone.

J. B. Reeside, Jr., Oscar Mueller, and

R. W. Imlay, 1944. NWJ of sec. 16,

T. 14 N., R. 21 E., Fergus County,

Mont. From 8 ft. of sandy limestone 14 ft. above base of Rierdon

formation, Gowericeras subitum beds.

No. on

fig. 1

12.__

13.__

14 _

14.. _

14...

15 _

16...

17...

18.__

ISIS—

19_._

20...

Geological Survey Mesozoic

locality

11312

19628

19629

20360

20363

19618

1163

1130

17106

17179

19632

21522

20974

Collector, year of collection, description of locality, and stratijtraphic

assignment

Frank Reeves, 1922. North side of

Button Butte, Sec. 18, T. 14 N., R.

24 E., Fergus County, Mont Rierdon

formation, Gowericeras subitum zone.

William Saalfrank and 'Carl s Rogers,

1945. Near line of sees. 2? and 33, 

T. 11 N., R. 21 E., Golden Valley

County, Mont. Rierdon formation,

15 to 20 ft. above base, Gcwericeras

beds.

Carl Rogers, William Saalfrank, and

R. W. Imlay, 1945. Sec. 36 T. 5 S.,

R. 24 E., Carbon County, Mont.

Rierdon formation, 23 ft. above base,

Kepplerites tychonis zone.

J. B. Reeside, Jr., and R. W. Imlay,

1946. Sec. 3, T. 24 E., R. 6 S.,

Carbon County, Mont. About 100

ft. below top of Rierdon formation,

Kepplerites tychonis zone.

J. B. Reeside, Jr., and R. W. Imlay,

1946. In the SEJ of sec. 4, T. 24 E.,

R. 6 S., Carbon County, Mont.

About 100 ft. below top of Rierdon

formation, Kepplerites tychonis zone.

R. W. Imlay and William Saalfrank,

1945. Sec. 36, T. 8 S., R. 7 E., Cinnabar Mountain, Park County, Mont.

Eight feet abqve base of Rierdon

formation, Arcticoceras codyense zone.

W. H. Weed, Summit of Wagon road between Sentinel Butte and Terrace

Mountain, Mammoth Hot Springs,

Yellowstone National Park. Rierdon

formation, Arcticoceras codyense

zone.

J. P. Iddings. Divide between Fawn

Creek and Gallatin Valley, Yellowstone National Park. Rierdon formation, Arcticoceras codyense zone.

D. A. Andrews, 1935. Soutl side of

Shoshone River 2 miles west of Cody,

Wyo. Base of Sundance formation

directly above red beds. Arcticoceras

codyense zone.

W. G. Pierce, 1935. South side of

Shoshone River 2 miles west of Cody,

Wyo. About 15 ft. above 48 ft. of

gypsiferous red beds that underlie

the Sundance formation.

R. W. Imlay and William Saalfrank,

1945. South side of Shoshone River,

2 miles west of Cody, in the NE£ of

sec. 4, T. 52 N., R. 102 W., Park

County, Wyo. From 15 to 20 ft.

above base of Sundance formation.

R. K. Hose and J. L. Weitz, 1949. Long.

110°32'24", Lat. 44°14'28", 2| miles

SW of Heart Lake, Ye^owstone

National Park. "Lower Sundance"

30 ft. below top.

R. W. Imlay, 1947. North side of

lower lake on the Gros Vent re River,

sec. 4, T. 42 N., R. 114 W., Teton

County, Wyoming. "Lower Sundance" about 200 ft. belovr top, or

about 30 ft. below lowest apnearance

of numerous Gryphaea nebrascensis

Meek and Hayden.
WESTERN INTERIOR UNITED STATES 17

Localities at which ammonites were collected from Callovian beds in the western interior of the United States—Continued

No. on 

fig. 1

21...

22...

22...

22 _

Geological Survey Mesozoic

locality

21635

20496

20497

20503

Collector, year of collection, description of locality, and stratigraphic

assignment

R. W. Imlay and G. Pipiringos, 1949.

East side of Red Creek, in the NE|

of sec. 7 and SEi of sec. 6, T. 6 N., R.

3 W., Fremont County, Wyo. From

12 to 18 feet below top of Canyon

Springs sandstone member of the

Sundance formation, Arcticoceras

codyense zone.

J. D. Love et al, 1946. Center of Ef 

of sec. 11, T. 29 N., R. 68 W., Platte

County, Wyo. Sundance formation,

Canyon Springs sandstone member,

Arcticoceras codyense zone.

J. D. Love et al, 1946. In the SE| of

sec. 11, T. 29 N., R. 68 W., Platte 

County, Wyo. Sundance formation,

Canyon Springs sandstone member,

Arcticoceras codyense zone.

J. D. Love et al, 1946. Center of Ef

of sec. 11, T. 29 N., R. 68 W., Platte

County, Wyo. Sundance formation,

Canyon Springs sandstone member,

Arcticoceras codyense zone.

No. on 

fig. 1

23_. _

24_._

24_._

25_._

Geological Survey Mesozoic

locality

20337

19560

19561

19556

Collector, year of collection, desc4ption of locality, and stratigraphic

assignment

J. B. Reeside, Jr., T. Botinelly, R. W.

Imlay, and G. Pipiringos, 1946 On

butte in sec. 23, T. 48 N., R. 62 W.,

Weston County, Wyo. From Canyon

Springs sandstone member of Sundance formation just above missive

gypsum. Arcticoceras codyense zone.

R. W. Imlay and William Saal'rank,

1945. In the SJ of sec. 19, T. 4 S.,

R. 1 E., Custer County, Dak. Basal 

5 feet of Stockade Beaver shale member of the Sundance formation,

Arcticoceras henryi beds.

R. W. Imlay and William Saal crank,

1945. In the NW| of sec. 8, T. 4 S.,

R. 1 E., Custer County, Dak. Basal 

5 feet of Stockade Beaver shale member of the Sundance formation,

Arcticoceras henryi beds.

R. W. Imlay and William Saal crank,

1945. In the Si of sec. 21, T. 7 S.,

R. 4 E., Fall River County, Dak.

From ( feet of shale containing nodular limestone about 48 feet below

top of Stockade Beaver shale member

of the Sundance formation, Arcticoceras henryi beds.

SUMMARY OF RESULTS

1. The described Callovian ammonites from the 

western interior of the United States include 10 

genera and 38 species and varieties. Of these, 19 

species and 1 variety are described as new. Of 

the total number of ammonite specimens, Arcticoceras comprises 20 percent, Cadoceras 28 percent, 

Kepplerites 15 percent, Gowericeras 23 percent, 

Cosmoceras 8 percent, and Xenocephalites, Procerites, and Grossouvria each about 2 percent. 

Only two specimens of Lilloettia have been found.

2. Most of the fossil collections made in recent years 

have been located closely stratigraphieally. This 

has been facilitated by the presence of several 

thin distinctive lithologie units that persist for 

scores or even hundreds of miles and change gradually in a predictable manner. These lithologic 

units have been discussed in detail in other publications by the writer and are merely summarized 

in this paper.

3. As a result of careful stratigraphic collecting, it 

has been possible to demonstrate the presence of 

five distinct ammonite zones in the marine Callovian, to show the positions of these zones with 

respect to the various lithologic units, and by considering both faunal and lithologic relationships 

to determine fairly accurately the history of Callovian time in the western interior region. The 

zones from the base upward are named after the 

characteristic species Arcticoceras codyense Imlay, Gowericeras costidensum Imlay, Gowericeras 

subitum Imlay, Kepplerites tychonis Ravn, and 

Kepplerites mclearni Imlay.

4. Comparisons of the ammonites in these zones with 

European ammonites show that the zones represent only about half of the Callovian stage. The 

basal zone of Arcticoceras codyense is correlated 

with the European zone of Macrocephalites macrocephalus mainly because of stratigraphic position. The Gowericeras costidensum and Gowericeras subitum zones are correlated with the 

European zone of Proplanulites koenigi because 

of the abundance of Gowericeras, the highest occurrence of Procerites, and the resemblance of 

several species to Gowericeras in the Kellaways 

beds of Great Britain. The overlying Kepplerites 

tychonis zone has been identified elsewhere in 

Alaska and East Greenland where associated species of Cosmoceras (Gulielmiceras) indicate an 

age not older than the European Sigaloceras calloviense zone. The highest zone of Kepplerites 

mclearni is correlated with the upper part of the 

European Sigaloceras calloviense zone because of 

the first appearance of Gulielmiceras and Gulielmites, the presence of Xenocephalites, the abundance of Kepplerites, and the characteristically 

flattened venters of the species of Kepplerites. 

This feature in Alaska is common in Kepplerites 

occurring in beds containing Gowericeras and
18 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

Paracadoceras, which are considered to be older 

than the Cosmoceras jason zone of Europe.

5. The Callovian ammonites of the western interior 

of the United States bear much greater resemblance to those of East Greenland than to those of 

southwestern Alaska. None of the subgenera of 

Cadoceras found in Alaska occur in the western 

interior and all the species of Cadoceras in the 

western interior belong to a subgenus that is unknown in Alaska but present in Greenland. Gowericeras and Gulielmiceras are much more common in the western interior than in Alaska. Lilloettia and Xenocephalites are fairly common in 

Alaska and are rare in the western interior. 

Paradoceras, Pseudocadoceras, Phylloceras, and 

Kheraiceras are very common in Alaska and unknown in the western interior. Nautiloid cephalopods occur in the Callovian of the western interior 

and not in Alaska. Arcticoceras is common at the 

base of the Callovian deposits of the western interior of North America, in northern Alaska, and 

in the Boreal region in general, but has not yet 

been found in the Alaska Peninsula or Cook Inlet 

regions of Alaska. Most of these differences appear to be ecological. Lithologic and faunal evidence indicates that the Callovian seas of the 

western interior and of East Greenland were very 

shallow and bordered lowlands, whereas the Callovian sea in the area of southwestern Alaska was 

fairly deep and bordered mountainous shores. 

Evidently only part of the ammonite genera that 

inhabited the deeper waters of the Boreal sea 

managed to become adapted to the shallower and 

probably warmer waters covering the interior of 

North America.

6. The distribution and associations of the Callovian 

ammonites in the western interior sea indicates 

that certain genera could live in much shallower 

water than others. It appears that Arcticoceras 

could live in shallow waters nearly to the strand 

line, that Gowericeras, Cadoceras, Procerites, and 

Grossouvria could not live in as shallow waters as 

Arcticoceras, and that Kepplerites preferred still 

deeper waters than Gowericeras. The absence of 

Cadoceras in the Kepplerites mclearni zone may 

be related to a shallowing and possible warming 

of the early Callovian sea during its withdrawal 

northward or due to the extinction of the single 

subgenus that managed to migrate into the western interior sea from the Boreal region. That no 

part of the western interior sea was deeper than 

the upper part of the neritic zone, and probably 

not deeper than 300 feet, is shown by the general

association of the ammonites with abundant 

Gryphaeas and a few Lingulas.

SYSTEMATIC DESCRIPTIONS

Genus LILLOETTIA Crickmay 1930

Lilloettia sp. ind. 

Plate 1, figures 10-13

The genus Lilloettia is represented in the Geological Survey collections from Montana by two septate 

specimens of vastly different sizes. Assignment to 

Lilloettia is based on the extremely small umbilicus, 

and the characteristics of the ribs, which thicken 

considerably ventrally and disappear anteriorly on 

the body whorl. Both specimens are compressed and 

have a narrowly rounded venter similar to tl at of 

L. lilloetensis Crickmay (1930, p. 62, pi. 18, figs. 1-4) 

but are much more coarsely ribbed than that species.

On the small specimen the ribbing is strong, gently 

flexuous and inclined slightly forward. The primary 

ribs are sharp, and bifurcate near the top of the 

lower third of the flanks. Many of the furcation 

points are indistinct. Most pairs of forked ril^ are 

separated by short intercalated ribs. The secondary 

ribs are stronger and rounder than the primary ribs 

and attain their greatest strength on the venter. 

They arch forward distinctly on the venter. The 

suture line is poorly preserved but is similar to that 

on specimens of Lilloettia of comparable size from 

Alaska.

The large specimen, which probably represents 

the penultimate whorl, bears thick, low, vT idely 

spaced ribs on its venter on the posterior part only. 

These ribs fade out dorsally near the middle of the 

flank. The entire anterior half of the septate whorl 

appears to be completely smooth. The suture line 

cannot be traced with certainty.

Probably the two specimens represent the same 

species, but this cannot be proven until better specimens are obtained. The ribbing on the small specimen resembles that of the coarsest ribbed species of 

Lilloettia from Alaska. The ribbing on the large 

specimen is much coarser than that of any l*nown 

species from Alaska or British Columbia.

Figured specimens, U.S.N.M. 108236, 108237.

Ellis group, Rierdon formation, Kepplerites mclearni beds, U.S.G.S. Mes. Iocs. 19583 and 19584.

Genus XENOCEPHALITES Spath 1928 

Xenocephalites phillipsi Imlay, n. sp.

Plate 1, figures 1, 2, 5, 9

The two available specimens of this species show 

the characteristics of only the body whorl and part 

of the penultimate whorl. Whorl section sul ovate,
WESTERN INTERIOR UNITED STATES 19

wider than high, widest below the middle of the 

flanks, embracing nearly completely; flanks gently 

convex, rounding rather abruptly into umbilicus, 

venter broadly rounded. Umbilicus very narrow, enlarging somewhat at anterior end of body whorl; 

wall low and steep. Body chamber incomplete but 

represented by at least five-sixths of a whorl.

The ribs on the penultimate whorl are fairly sharp, 

low, a little narrower than the interspaces, strongly 

flexuous, and generally bifurcate on the lower third 

of the flanks. Most pairs of forked ribs are separated 

by short, intercalated ribs. On the body whorl the 

ribbing rapidly becomes higher and more widely 

spaced anteriorly and intercalated ribs become less 

common.

The suture line illustrated is the next to the last 

preceding the body chamber and is very similar to 

that of some forms of Cranocephalites figured by 

Spath (1932, pi. 5, figs. 7, 8). The first lateral lobe 

is as long as the external lobe, the second lateral lobe 

is much shorter, and the greatest thickness of the 

shell coincides with the inner part of the first lateral 

saddle. Comparisons with the suture line of Xenoeephalites borealis Spath (1932, p. 44, pi. 14, figs. 

4a-d) are not useful because of the differences in the 

sizes of the specimens. The simple suture contrasts 

markedly with that of Kheraiceras, whose ornamentation and form resembles Xenocephalites.

The holotype at a diameter of 56 mm has a whorl 

height of 28 mm and a whorl thickness of 34 mm. 

These measurements were made just back of the 

crushed anterior end of the body chamber.

This species has much denser, finer ribbing than 

X. bearpawensis Imlay and is rather similar to species in the Chinitna formation of the Cook Inlet region, Alaska. X. nikitini (Burckhardt) (1927, p. 33, 

pi. 16, figs. 4-9), from southern Mexico, has sparser 

but weaker ribbing. X. parvus var. angustumbilicata 

(Stehn) (1924, p. 92, pi. 1, fig. 6, text. fig. 13) appears to have finer ribbing on the body chamber and 

higher points of rib branching.

Holotype, U.S.N.M. 108238; paratype, U.S.N.M. 

108239.

Ellis group, Rierdon formation, Kepplerites mclearni beds, U.S.G.S. Mes. loc. 10730.

Xenocephalites bearpawensis Imlay, n. sp.

Plate 1, figures 3, 4, 6, 8

Six specimens of this species are at hand. The 

holotype has been crushed laterally but the two figured paratypes appear to be undeformed. Whorls 

ovate in section, higher than wide, widest a little 

below the middle of the flanks, embracing nearly

completely; flanks gently convex; venter everly 

arched. Umbilicus very narrow, enlarging slightly 

at anterior end of body whorl; wall low and routing evenly into flanks. Body chamber represented by 

about three-fifths of a whorl. The aperture is probably complete in the specimen shown on pi. 1, fig. 6.

The ribbing, exposed only on the body whorl, is 

high, sharp ventrally, very widely spaced, flexuous, 

inclined forward on the flanks, and arched forward 

gently on the venter. Nearly all the primary ribs 

bifurcate slightly below the middle of the flanks into 

sharper and narrower secondary ribs. Intercalated 

ribs are uncommon.

The suture line cannot be traced.

The largest paratype (pi. 1, fig. 6) at a diameter 

of about 64 mm has a whorl height of 29 mm and 

an umbilical width of 8 mm.

This species has the strongest and most widely 

spaced ribs of any known form of Xenocephalites. 

X. neuquensis (Stehn) (1924, pp. 86-88, pi. 1, fig. 3, 

text fig. 11) has the most similar ribbing but has a 

lower whorl section and an umbilicus that enlarges 

rapidly at the anterior end of the body whorl.

Holotype, U.S.N.M. 108240; paratypes, U.S.NJT. 

108241, 108242.

Ellis group, Rierdon formation, Kepplerites melearni beds, U.S.G.S. Mes. Iocs. 10730, 10732, and 

18758.

Xenocephalites cf. X. nikitini (Burckhardt) 

Plate 1, figure 7

One small fragment has blunter ribbing than any 

of the known species of Xenocephalites from the 

western interior or from Alaska but is rather similar 

to X. nikitini (Burckhardt) (1927, p. 33, pi. 16, fig-a. 

7-9) from the Callovian of southern Mexico. It do^s 

not show thickening of the ribs on the venter as in 

Lilloettia. In Kheraiceras rib-branching occurs 

higher on the flanks, and specimens of comparable 

size are much more densely ribbed.

Figured specimen, U.S.N.M. 108243.

Ellis group, Rierdon formation, Kepplerites mclearni beds, U.S.G.S. Mes. loc. 18737.

Genus ARCTICOCERAS Spath 1932 

Arcticoceras rierdonense Imlay, n. sp. 

Plate 2, figures 1, 2; plate 3, figures 16, 18, 21, 22 

This species is represented in the Survey colle-.- 

tions by eleven specimens, and probably also IT 

sever*al large fragments and several immature specimens. Form lenticular, compressed. Whorls elliptical, much higher than wide, thickest near tH 

umbilicus. Flanks flattened,- converging gently to
20 CALLOVIAN (JUEASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

rather narrowly rounded venter. Umbilicus nearly 

closed. Body chamber represented by about threefifths of a whorl. Aperture on internal mold slightly 

constricted, inclined forward in a gently sinuous 

manner, preceded near umbilicus by a broad, shallow 

constriction.

Certain small specimens shown on pi. 3, figs. 19, 

20, are interpreted as the immature forms of A. rierdonense as they were obtained from the same localities as some of the larger specimens. These small 

specimens have flexuous ribbing that curves forward 

on the upper part of the flanks and on the venter. 

The primary ribs are fairly sharp and bifurcate at 

about the top of the lower third of the flanks into 

weaker secondaries. Most pairs of forked ribs are 

separated by single intercalated ribs that begin along 

the zone of furcation.

During growth, eifacement of the ribs takes place 

on the lower parts of the flanks and gradually 

spreads ventrally. The ribs on the upper parts of 

the flanks and on the venter are moderately strong, 

strongly curved forward, and extend to the anterior 

end of the septate shell. The body chamber is nearly 

smooth except for faint, broad undulations on the 

venter.

The suture line, well exposed on several specimens, 

has fairly slender lobes and saddles on the immature 

forms, but these sutural elements become broad and 

shallow on the last septate whorl (pi. 3, fig. 1). The 

second lateral lobe is irregularly trifid.

Most of the specimens have been compressed or 

distorted. However, the anterior half of the paratype shown on plate 3, figures 21, 22, appears to be 

nearly undeformed. At a diameter of 94 mm, it has 

a whorl height of 55 mm and a whorl thickness of 

38mm.

Compared with Arcticoceras kochi Spath (1932, p. 

53, pi. 12, fig. 1; pi. 13; figs. 4, 5; pi. 14, figs. 1-3; pi. 

15, fig. 1) from Greenland, the present species has 

finer ribbing that begins to disappear much earlier 

and is. curved forward less strongly on the upper 

part of the flanks. Adult specimens are very similar, 

but A. kochi has a deep constriction near the aperture. A. ishmae (Keyserling) (Sokolov, 1912, pp. 15, 

49, pi. 1, fig. 1, pi. 3, fig. 12; Spath, 1932, pi. 15, figs. 

7a, b; Roman, 1938, pi. 20, fig. 204) from Petchora 

land has finer and less projected ribbing than A. 

kochi but much coarser ribbing than A. rierdonense. 

Both of the Boreal species appear to have a more 

robust form and a wider umbilicus than A. rierdonense. A. codyense Imlay has a more trigonal-shaped 

whorl section, finer ribbing that disappears at an 

earlier growth stage, and a bifid second lateral lobe.

A. henryi (Meek and Hayden) (1865, pp. 123, 124, 

pi. 5, figs. 9a-c) has a much stouter form and loses 

its ribbing at a much smaller size.

Holotype, U.S.N.M. 108244; paratypes, U.S.N.M. 

108245a, b, 108246.

Ellis group, Rierdon formation, Articoceras beds, 

U.S.G.S. Mes. localities 1130, 18610, 19397, 19593, 

and 19605.

Arcticoceras n. sp. ind. 

Plate 3, figures 13, 15, 17

One species is represented by six fragmentary 

specimens, none sufficiently well preserved to merit 

designation as a type. The species is characterized by 

a trigonal whorl section and by very broad, low, 

widely-spaced ribs. The ribs disappear f^om the 

lower part of the flanks at a small diameter but persist on the venter as broad swells to diameters of at 

least four inches. The ribbing is as widely spaced as 

in A. crassicostatum Imlay but much weaker. A. 

codyensis Imlay has a similar whorl section t fit much 

finer ribbing.

Figured specimens, U.S.N.M. 108248, 108349.

Ellis group, Rierdon formation, Arcticoceras beds, 

U.S.G.S. Mes. Iocs. 1163, 18610,19600, 19605, 20974, 

and probably 19618.

Arcticoceras crassicostatum Imlay, n. sp. 

Plate 3, figures 1-6

Four specimens of this species are known and 

these appear to be nearly undeformed. It seems unlikely that they represent adults. Form compressed. 

Whorls subovate, higher than wide, thickest on the 

lower third of flanks. Flanks slightly convex, converging above into narrowly rounded venter. Umbilicus narrow but wider than in most species of 

Arcticoceras; wall low, vertical at base, rounding 

evenly into flanks. Body chamber incomplete, but 

represented by at least half a whorl.

The ribs, known only from the body wl orl, are 

very prominent, widely spaced, and flexuo'is. The 

primary ribs are radial on the umbilical wall and incline forward on the lower part of the flank. About 

half of them bifurcate at or just below the top of the 

lower third of the flank. Some of the furcation points 

are so indistinct as to give the appearance of alternation of long and short ribs. The secondary ribs bend 

backward slightly near the middle of the fla nks and 

then strongly forward on the upper part of ti'« flanks 

and on the venter. Along the midventral line the anterior slope of the ribs is appreciably steeper than 

the posterior slppe.

The suture line is barely visible.
WESTERN INTERIOR UNITED STATES 21

The holotype at a diameter of 56 mm has a whorl 

height of 25 mm, a whorl thickness of 19 mm, and 

an umbilical width of 10 mm. As the specimen has 

been deformed by lateral compression, the dimensions should be considered only as approximate.

This species has coarser ornamentation and a 

wider umbilicus than any other species of Arcticoceras. When first examined it was thought to represent 

the inner whorls of some coarsely-ribbed Arctocephalites, such as figured by Imlay (1948, p. 19, pi. 6, figs. 

2, 5). However, its venter is more narrowly rounded 

than in Arctocephalites and the pronounced forward 

arching of the ribs on the venter contrasts with the 

transverse ribbing on Arctocephalites. Also, this 

species resembles both Arcticoceras ishmae (Keyserling) (see Spath, 1932, pi. 15, figs. 7a, b) from the 

Boreal region and A. loveanum Imlay with which it 

is associated, differing from them mainly in its more 

compressed form and stronger ribbing.

Holotype, U.S.N.M. 108250; paratypes, U.S.N.M. 

108251a, b.

Sundance formation, Canyon Springs sandstone 

member, Arcticocera's beds, U.S.G.S. Mes. Iocs. 20496 

and 20497.

Arcticoceras loveanum Imlay, n. sp. 

Plate 3, figures 7-12, 14

Ten specimens of this species are available, but 

probably none is an adult. Most of them are slightly 

crushed. Whorls ovate, probably a little higher than 

wide before crushing. Flanks gently convex, rounding evenly into umbilicus, converging to narrowed 

venter. Umbilicus very narrow. Body chamber incomplete, represented by at least half a whorl.

The ribs are moderately strong, incline forward 

gently on the lower half of the flanks, curve forward 

strongly on the upper half of the flank, and form a 

distinct forward arch on the venter. The primary 

ribs bifurcate between the tops of the lower fourth 

and lower third of the flanks. Some of the furcation 

points are indistinct and there are a few intercalated 

ribs that begin along the zone of furcation. Anteriorly on the body chamber the ribbing coarsens rapidly and becomes widely spaced. The ribs along the 

midventral line have steeper anterior than posterior 

slopes.

The suture line is barely visible in a few places and 

cannot be traced. Owing to crushing, measurements 

of dimensions are useless.

This species differs from Arcticoceras crassicostatum Imlay in its considerably finer ornamentation 

and more inflated shape. The strength and spacing 

of its ribbing resemble that of the septate shell of

the much larger Arcticoceras ishmae (Keyserling) 

(Pompeckj, 1899, p. 72; Sokolov, 1912, pp. 15, 49, 

pi. 1, fig. 1; Spath, 1932, pi. 15, figs. 7a, b; Romf.n, 

1938, pi. 20, fig. 204), which becomes smooth on the 

body whorl. This species is named in honor of J. D. 

Love of the Geological Survey, who was in charge of 

the field party that collected most of the specimens.

Holotype, U.S.N.M. 108252; paratypes, U.S.N.M. 

108253a, b, 108254.

Sundance formation, Canyon Springs sandstone 

member, Arcticoceras beds, - U.S.G.S. Mes. lo^s. 

20337,20496, and 20497; Sundance formation, Stockade Beaver shale member, basal 15 ft., Mes. loc. 

19556.

Arcticoceras eodyense Imlay 

Plate 4, figures 1-13

Arcticoceras eodyense Imlay, U. S. Geol. Survey Prof. Paper 

214-B, pp. 20, 21, pi. 6, figs. 4, 6, 8, 1948.

This species is the most common representative of 

Arcticoceras in the western interior, being represented by about forty specimens of various sizes. 

Most of the specimens have been deformed. Soire. 

including the holotype, have been compressed laterally, thereby producing a high, thin whorl section. 

Others have been depressed, resulting in a faifFy 

thick whorl section. The specimens from the Little 

Rocky Mountains of central Montana appear to be 

least deformed and are herein illustrated to show tne 

changes in whorl shape.

Shell lenticular, flattened in young, stout in adult. 

Whorls trigonal in section, much higher than wic1 ?, 

becoming stouter during growth, thickest near t!he 

umbilicus. Flanks flattened; venter narrowly rounded; umbilicus nearly closed. Body chamber incomplete but represented by at least half a whorl.

The small immature specimens have many low, 

flexous ribs separated by interspaces of about equal 

width. These ribs are indistinct on the lower parts 

of the flanks at diameters of about 25 mm. During 

growth effacement of the ribs spreads ventrally, resulting in a nearly smooth mold at diameters of about 

60 mm. The secondary ribs curve forward strongly 

on the upper parts of the flanks, arch forward distinctly on the venter, and are thickest and most persistent on the venter. The body whorl of adult specimens shows no trace of ribbing.

The suture line, well exposed on the holotype and 

on several larger specimens, is characterized by a 

bifid second lateral lobe tliat contrasts with the suture of A. rierdonense Imlay. The general sutural 

pattern is similar to that of A. henryi (Meek and 

Hayden), differing mainly in its broader saddles.
22 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

The dimensions of the types in millimeters and in 

ratios of whorl height and thickness to the diameter 

are as follows i 1

Specimen

Holotype (pi. 4 

Paratype (pi. 4 

Paratype (pi. 4

, figs. 1, 4)..._ 

, figs. 9, 12).. _ 

, fig. 13) — -..

Diameter

85 

88 

181

Whorl 

height

51 (0.60) 

52 (0.59) 

110 (0.60)

Whorl 

thickness

36 (0.42) 

36 (0.40) 

91 (0.50)

The largest specimen, collected from Chalk Butte 

dome in the Little Rocky Mountains, is about 280 

mm in diameter. It has only half the whorl of body 

chamber and lacks the aperture. However, a weak 

constriction similar to that near the aperture of the 

adult of A. rierdonense (pi. 2, fig. 2) suggests that 

the body chamber is nearly complete. This specimen 

of Arcticoceras codyense maintains its trigonal 

shape on the body chamber and is thus readily differentiated from the similar large adult specimens of 

A. rierdonense.

Arcticoceras codyense is characterized by its trigonal whorl shape, bifid second lateral lobe, rather 

fine ribbing, and by its loss of ribbing at a smaller 

growth stage than any of the described species of 

Arcticoceras except A. henryi (Meek and Hayden). 

The latter has a much thicker, rounder whorl section 

and finer, sparser ribbing. The other species are not 

closely comparable.

Holotype, U.S.N.M. 104132; paratypes, U.S.N.M. 

104133 a-c; plesiotypes, U.S.N.M. 108255, 108256, 

108257 a, b.

Ellis group, Rierdon formation, Arcticoceras beds, 

U.S.G.S. Mes. Iocs. 18718, 19588, 19589, and 19600; 

Sundance formation, basal limestone at Mes. Iocs. 

17106 and 21635; Sundance formation, Canyon 

Springs sandstone member at Mes. Iocs. 20337, 

20496, 20497, and 20503.

Arcticoceras henryi (Meek and Hayden)

Plate 5, figures 6-15

Ammonites henryi Meek and Hayden, Smithsonian Contr. 

Knowledge, vol. 14, Art. 5, pp. 123, 124, pi. 5, figs. 

9 a-c, 1865.

Arcticoceras henryi (Meek and Hayden), Imlay, U. S. Geol. 

Survey Prof. Paper 214-B, p. 21, 1948.

The Survey collections contain twenty-two speciments of this species. Most of these are undef ormed. 

Shell lenticular, fairly stout. Whorls ovate in section, higher than wide, thickest at the top of the 

lower third of the flanks. Flanks gently convex, be-

!In the following pages this statement will be abbreviated to "Dimensions 

in millimeters and ratios are as follows:"

coming more convex on body chamber. Venter highly 

arched. Umbilicus nearly closed; wall low, vertical 

at base, rounding evenly into flanks. Body chamber 

represented by three-fourths of a whorl. Aperture 

sinuous, inclined strongly forward, and produced 

ventrally in a long lappet.

The smallest specimens, ranging from 35 to 50 

mm in diameter, have striae on the lower parts of 

the flanks and weak, widely-spaced ribs on the ventral region. These incline forward on the lorrer third 

of the flank, then recurve slightly, curve forward 

strongly on the upper parts of the flanks, and arch 

forward on the venter. This ornamentation weakens 

rapidly anteriorly and at diameters greater than 50 

mm the internal molds are nearly smooth. Viewed 

under oblique lighting the larger specimens show 

faint, sparse undulations and even fainter striae that 

project forward on the ventral region.

The suture line, well preserved on many sj «cimens, 

has rather broad saddles and lobes. Its mcTt interesting feature is the second lateral lobe, which is 

trifid on immature specimens but becomes bifid on 

most adults by diminution of the innermost lobule. 

On the holotype the second lateral lobe is broad and 

bifid on one side of the shell, but narrower snd trifid 

at the same position on the other side of the shell.

Dimensions in millimeters and in ratio'* are as 

follows:

Specimen

Holotype (pi. 5 , fig. 11) _._._.

Plesiotype (pi. 5 , figs. 13, 15). 

Plesiotype (pi. 5 , figs. 8, 9)__-

Diameter

79

93

54

Whorl 

height

44 (0.55)

53 (0.57) 

32 (0.59)

Whorl 

thickness

?

45 (0.48) 

28 (0.52)

This species has the weakest ribbing of any known 

species of Arcticoceras. Its venter is conriderably 

broader than A. rierdonense Imlay and is comparable with that of A. ishmae (Keyserling), as figured 

by Spath (1932, pi. 15, fig. 7b). Its smooth lateral 

aspect resembles that of the adult of Cadoceras subpatruum Nikitin (1885a, p. 58, pi. 11 (13), fig. 58), 

whose inner whorls are reported to be ribbed as in 

C. surense Nikitin (1885a, p. 57, pi. 10 C.2), figs. 

53, 54a, b, 55) and to have comma-shaped tubercles. 

However, on C. subpatruum the trigonal whorl section, the very narrow umbilicus; the f orwarc1 arching 

of the ribs on the ventral region, the thickening of 

the ribs along the midventral line, and the complete 

disappearance of ornamentation except for fine 

striae on the body chamber are all characteristics
WESTERN INTERIOR UNITED STATES 23

of Afcticoceras. Also, the occurrence of C. subpatruum in the Macrocephalites beds of Russia agrees 

with the stratigraphie occurrence of Arcticoceras, 

whereas similarly compressed forms of Cadoceras 

are characteristic of higher beds in the middle part 

of the Callovian.

Holotype, U.S.N.M. 314; plesiotypes, U.S.N.M. 

108258a-f, 108259.

Sundance formation, Stockade Beaver shale member, basal 15 feet, top of Arcticoceras beds, U.S.G.S. 

Mes. Iocs. 19556, 19560 and 19561.

Genus Cadoceras Fischer 1882

Cadoceras shoshonense Imlay

Plate 6, figures 1-14

Cadoceras shoshonense Imlay, U. S. Geol. Survey Prof. Paper 

214-B, p. 22, pi. 7, figs. 13, 16, 17, 1948.

The species is represented in the collections by at 

least 46 specimens. It probably includes two large, 

globose specimens measuring about 8 inches in diameter, but the ornamentation of their inner whorls 

could not be determined. The species was originally 

described as follows:

Form globose, tending to become broader and more spherical during growth. Whorl semicircular in section, wider than 

high, becoming wider during growth, and embracing most of 

the preceding whorl. Flanks rounding evenly into the broadly 

rounded venter. Umbilicus rather narrow; wall vertical at 

base, steep above, and rounding rather abruptly into flanks. 

Body chamber apparently nearly complete on the holotype 

where it comprises three-fourths of a whorl.

The ribs are coarse and high. On the inner whorls they are 

as wide as the inner spaces, but during growth become narrower than the inner spaces. The primary ribs are faint on 

the upper part of the umbilical wall and are weak on the 

edge of the umbilicus, but become strong ventrally. They 

curve forward considerably on the lower fourth of the flank 

and give rise to pairs of secondary ribs that incline forward 

slightly on the flanks and arch forward gently on the venter. 

Occasional intercalary ribs begin just above the zone of 

furcation.

The last suture line of the holotype is characterized by the 

shortness of its lobes and saddles. This may reflect the proximity of the body chamber, although a smaller specimen 

(paratype, U.S.N.M. 104144) has several suture lines that 

are similarly short, and which contrast with those of the 

other species of Cadoceras in the Gowericeras beds. Comparable short sutural elements occur on C. tschernyschewi 

Sokolov (1912, pp. 19, 51, pi. 1, figs. 2a, b, pi. 2, fig. 1).

Cadoceras shoshonense is distinguished by its 

globose form, coarse ribbing and small umbilicus. 

Among the associated species, Cadoceras muelleri 

Imlay has a similar form but its ribbing is considerably finer. C. piperense Imlay approaches it in 

coarseness of ribbing but has a much more compressed form. None of the described species from 

Alaska or Canada closely resemble it. C. freboldi

Spath (1932, p. 65, pi. 18, figs. 2a, b) from East 

Greenland has a different whorl shape and finer ribbing. Cadoceras? sp. in. Spath (1932, p. 70, pi. 20, 

fig. 5) resembles C. shoshonense in whorl shape and 

umbilical width, but its ribbing is different. The 

rather blunt ribbing of C. shoshonense, especially in 

the young forms, clearly distinguishes it from Pleurocephalites.

Dimensions in millimeters and in ratios of the 

diameter are as follows:

Specimen

Paratype (pi. 6, figs. 

7,11) 

Paratype (pi. 6, figs. 

3,4) 

Holetvpe (pi. 6, figs. 

13, 14) 

Paratype (pi. 6, fig. 

12) 

C. cf. C. skoshonense..

Diameter

41 

53 

74 

90 

205

Whorl 

height

21 (0.51) 

29 (0.54) 

30 (0.40) 

38 (0.42) 

100 (0.48)

Whorl 

thickness

30 (0.73) 

37 (0.69) 

47 (0.63) 

58 (0.64) 

165 (0.80)

Umbrlieal

10 (0.24) 

12 (0.22) 

15 (0.20) 

19 (0.21) 

30 (0.14)

Types.—Holotype, U.S.N.M. 104143; paratypes, 

U.S.N.M. 104141, 104144a-c; plesiotypes, U.S.I T.M. 

104142, 104145a, b, 108260, 108261a, b.

Ellis group, Rierdon formation, Gowericeras fubitum beds, U.S.G.S. Mes. localities 11312, 18718, 

19169, 19179-19182, 19202,19607,19628, and 22-51; 

Gowericeras costidensum beds, U.S.G.S. mes. localities 19170, 19212, and 19585; Sundance formation, 

lower part, Mes. localities 17179, 19632, 21522.

Cadoceras muelleri Imlay, n. sp. 

Plate 7; plate 8, figures 2, 7, 9; plate 9, figures 1-3, 6-8, 10

Forty-five specimens, representing most stages of 

growth, are at hand. Form globose. Whorls semicircular in section, much wider than high, embracing most of the preceding whorl. Flanks rounding 

evenly into the broadly rounded venter. Umbilicus 

fairly narrow and deep; wall vertical, rounding 

rather abruptly into flanks. Adult body chamber 

represented by at least five-sixths of a whorl, slightly 

contracted at anterior end. Aperture not preserved.

The ribs are moderate in height, fairly thick, as 

wide as the interspaces on the inner whorls, becoming weaker and more widely separated anteriorly. 

On the smallest specimen (pi. 9, figs. 2, 6) the ribs 

are faint on the lower fourth of the flanks, but there 

are indications of rib branching at about the top of 

the lower fourth. On the upper three-fourths of the 

flanks the ribs incline forward slightly and then 

cross the venter with a gentle forward arch. Effaeement of the ribs spreads ventrally during growth,
24 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

resulting in a completely smooth body whorl on the 

largest adults.

The suture line has moderately long lobes and saddles. On one small specimen (pi. 9, fig. 7) the first 

lateral lobe is shorter than the external lobe and the 

second lateral saddle is not entirely outside the umbilical edge. On some large specimens the first lateral lobe is as long as the external lobe and the second lateral saddle is removed from the umbilical 

edge.

Dimensions in millimeters and ratios are as follows:

Specimen

Paratype (pi. 9, figs.

2,6)

Paratype (pi. 9, figs.

1,3)

Holotype (pi. 8, fig.

7)

Holotype (pi. 8, fig.

7).

Diameter

54

82

124

190

Whorl 

height

32 (0.59)

48 (0.58)

65 (0.52)

81 (0.42)

Whorl 

thickness

42 (0.78)

73 (0.89)

?

?

Umbilical 

width

10 (0.18)

16 (0.19)

19 (0.13)

40 (0.21)

The species attains a larger size than the above 

dimensions show as the smooth body whorl of the 

holotype is about 290 mm in diameter and there are 

fragments suggesting even larger specimens.

This species differs from Cadoceras shoshonense 

Imlay mainly in its much finer, closer-spaced ribbing 

that begins to disappear at an earlier growth stage 

and by the longer elements of its suture line. The 

effacement of the primary ribs contrasts with the 

rather strong primaries on C. freboldi Spath (1932, 

p. 65, pi. 18, figs. 2a, b) and Cadoceras? sp. ind. 

Spath (1932, p. 70, pi. 20, fig. 5), which are otherwise rather similar to C. muelleri. This species is 

named in honor of Oscar 0. Mueller, a lawyer and 

ardent amateur collector, of Lewistown, Montana.

Holotype, U.S.N.M. 108262; paratypes, U.S.N.M. 

108263-108266.

Ellis group, Rierdon formation, Gowericeras beds, 

U.S.G.S. Mes. Iocs. 19169,19170,19175,19179-19182, 

19202, 19585, 19587, 19591, 19596, 19607, 19629 and 

20365; Sundance formation, near base, Mes. loc. 

19632.

Cadoceras tetonense Imlay, n. sp. 

Plate 10, figures 1, 2; plate 11, figures 1-10 

Twelve specimens of this species are available. 

Shell stout in young, globose in adult forms. Whorls 

semicircular in section, much wider than high, embracing most of the preceding whorl. Flanks rounding evenly into the broadly rounded venter. Umbilicus fairly narrow and deep; wall vertical, reminding 

rather abruptly into flanks. Adult body chamber 

represented on holotype by a complete whorl whose 

anterior end is somewhat contracted. Aperture not 

perfectly preserved but showing a swelling that is 

preceded by a broad, weak constriction.

The ribs on the smallest specimens (pi. 11, figs. 1, 

3, 9) are fairly high and narrow on the venter and 

widely separated. They incline forward on the 

flanks and arch forward gently on the venter. The 

primary ribs are much broader and lower than the 

secondary ribs and pass into two or three secondary 

ribs at about the top of the lower fourth of the flanks. 

There are a few intercalated ribs.

At diameters greater than about 60 mm effacement of the ribs begins on the lower parts of the 

flanks and gradually spreads ventrally. Concomitantly the ribbing on the ventral region becomes 

lower, broader, and about as wide as the interspaces. 

The last half whorl of the body chamber is nearly 

smooth except for faint undulations on the venter.

The suture line is characterized by broad, short 

elements. The first lateral lobe is slightly shorter 

than the external lobe and the second lateral saddle 

is considerably outside of the umbilical edge.

Dimensions in millimeters and ratios are as follows :

Specimen

Paratype (pi. 11, figs.

1, 3, 9).

Paratype (pi. 11, figs.

5,6).

Paratype (pi. 11, figs.

8, 10).

Holotype (pi. 10, figs.

1,2).

Holotype (pi. 10, figs.

1, 2).

Diameter

36

56

89

130

160

Whorl 

height

18 (0.50)

25 (0.44)

53 (0.55)

59 (0.45)

79 (0.49)

Whorl 

thickness

21.5 (0.59)

32? (0.57)

68? (0.71)

104 (0.80)

134? (0.83)

Umbilical 

width

5.5 (0.15)

7 (0.12)

18 (0.18)

23 (0.17)

31 (0.19)

This species differs from the associated C. muelleri 

Imlay in its weaker, denser ribbing and by its less 

globose small and intermediate sized whorls. C. 

piperense Imlay is more compressed and its ventral 

ribbing is higher and more sparse. C. shoshonense 

has a similar suture" line but has much coarser ribbing. C. franciscus Spath (1932, p. 74, pi. 20, figs. 

2 a-c) resembles the immature forms of C. tetonense 

in ribbing and whorl shape, although it appears to 

have a wider umbilicus and sharper primary ribs.

Holotype, U.S.N.M. 108267; paratypes, U.S.N.M. 

108268, 108269-108271.
WESTERN INTERIOR UNITED STATES 25

Ellis group, Rierdon formation. Arcticoceras beds 

at U.S.G.S. Mes. Iocs. 19593 and 19594; Gowericeras 

beds at Mes. Iocs. 19169, 19179, and 19202; not localized stratigraphically at Mes. loc. 18718; Sundance 

formation, near base at Mes. loc. 19632.

Cadoceras piperense Imlay, n. sp. 

Plate 12; plate 13, figures 1-12; plate 14, figure 7

This species is represented by thirty specimens, 

including nine adults. Many of these are contorted. 

Shell compressed in young, stout in adult. Whorls 

ovate in section, a little higher than wide in immature specimens, becoming wider than high in adult, 

embracing most of the preceding whorl. Flanks 

gently convex, rounding evenly into moderately 

broad venter. Umbilicus fairly narrow but enlarging on anterior third of body whorl. Wall vertical, 

rounding rather abruptly into flanks. Body chamber 

represented by five-sixths of a whorl, contracted 

somewhat at anterior end. Aperture inclined forward, preceded on internal mold by a weak constriction.

The smallest known whorls at diameters of about 

25 mm have broad, fairly strong ribs on the venter 

and upper half of the flanks, but are nearly smooth 

on the lower fourth of the flanks. Some specimens 

bear faint indications of rib branching at about the 

top of the smooth area. During growth the ribs become much broader but at all growth stages are 

slightly narrower than the interspaces. The ribs 

incline forward slightly on the flanks and arch forward gently on the venter. Effacement of the ornamentation in the ventral area occurs gradually on 

the body whorl.

The suture line has fairly long lobes and saddles. 

The first lateral lobe is deeper than the external lobe 

and the second lateral saddle is barely outside of the 

umbilical edge.

Dimensions in millimeters and ratios are as follows :

Specimen

Paratype (pi. 13, figs.

5,6).

Paratype (pi. 13, figs.

1,2).

Paratype (pi. 13, figs.

3,4).

Holotype (pi. 14, fig.

7).

Diameter

33

51

60

145

Whorl 

height

16.5 (0.50)

29.5 (0.58)

30 (0.50)

81 (0.56)

Whorl 

thickness

15.5 (0.47)

29 (0.57)

32 (0.53)

98 (0.67)

Umbilical 

width

4.5 (0.14)

5.5 (0.11)

8 (0.13)

26 (0.18)

This species is the most compressed of any Cadoeeras in the collections from the western inter:'or. 

Its ventral ribbing is a little broader, lower and 

sparser than that of C. muelleri Imlay. The immature shells have dimensions similar to that of C. franciscus Spath (1932, p. 74, pi. 20, figs. 2 a-c), which is 

distinguished by a wider umbilicus and by stronger 

ribbing on the lower parts of the flanks. None of 

the Alaskan species is very similar.

Holotype, U.S.N.M. 108272; paratypes, U.S.N.M. 

108273-108278.

Ellis group, Rierdon formation, U.S.G.S. Mes. Iocs. 

18610, 18718, 19169, 19170, 19176, 19179-19181, 

19213, 19590, 19596, 19599, 19602, 19628; Sundance 

formation, lower part, Mes. Iocs. 19632 and 21635. 

The species occurs mainly in the zone of Gowericeras 

subitum. At Mes. loc. 21635, it is associated with 

Arcticoceras. Some of the collections from the Sawtooth Range, such as 18610 and 18718, are not localized and may be from the Arcticoceras zone.

Genus Kepplerites Neumayr 1892 

Kepplerites (Seymourites) mclearni Imlay 

Plate 15; plate 16; plate 17, figures 1-8 

Kepplerites (Seymourites) mclearni Imlay, IT. S. Geol. Survey 

Prof. Paper 214-B, pp. 24, 25, pi. 8, figs. 1, 2, 5, 7, 10; 

pi. 9, fig. 1, 1948.

This is the most common species in the Little 

Rocky Mountains. It is represented by 45 SDecimens, 

most of them fragmentary and conmressed, although 

the various growth stages are well shown. The immature specimens have a subauadrate whorl section 

about two-thirds as wide as high. During grov^h 

the whorls become relatively stouter, and in the adult 

the width is about five-sixths the height. The flanks 

are somewhat flattened, becoming less so duringgrowth. The venter is runcinate and slightly concave 

up to a diameter of !*/£ in. (38 mm), and some fattening is visible up to a diameter of 214 in. (60 mm). 

Thereafter, it changes to narrowly rounded, and, on 

the outer whorl is moderately rounded. The urnh'lical wall is low, rounds evenly into the flanks, is steeply inclined on the inner whorls, and is very gertly 

inclined on the outer whorls. The umbilicus is rarrow on the smallest whorls but widens rapidly at 

diameters of 4i/| to 51/9 in. and thereafter is wMe. 

The body chamber is completely preserved on the 

largest specimen (see pi. 16) and is represented by 

one and one-quarter volutions.

The ornamentation changes little up to the body 

chamber. On the smaller whorls (pi. 17, figs. 1, 4, 5) 

the ribs are sharp and moderately spaced. The primary ribs are strong, elevated, and nearly radial, or 

curve backward slightly on the umbilical wall then 

curve forward on the lower fourth of the flanks and
26 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

terminate in prominent, conical tubercles. The secondary ribs incline gently forward, are slightly flexuous at diameters of less than li/ij in., and cross the 

venter transversely. Bifurcation is the common 

mode of branching at all growth stages, but some 

trifurcation occurs at diameters greater than 2 

inches. Most rib branches are separated by single 

intercalary ribs which begin at or slightly above the 

line of tubercles. On the intermediate whorls the 

ribbing gradually becomes coarse, and the tubercles 

remain fairly conspicuous as far as the body whorl. 

On the body whorl the ornamentation gradually 

changes. The tubercles disappear, single ribs become 

more common, and effacement of the ribbing occurs, 

especially on the lower part of the flanks. The specimen shown on pi. 17, figs. 3, 6, has at a diameter of 

about 21/2 in. (60 mm) 25 primary ribs and about 

85 secondary ribs. The holotype (pi. 15) at a diameter of about 8 in. (125 mm) has 36 primary ribs 

and about 106 secondary ribs. At a diameter of 12 

in. (300 mm) it has 68 primary ribs and 142 secondary ribs.

The suture line is deeply dissected. Ventral lobe 

longer than wide; first lateral lobe a little longer 

than ventral lobe but narrower; second lateral lobe 

shorter than first lateral lobe. Saddles slightly wider 

than lobes; ventral saddle longer than wide and divided by a lobule, which is shorter than the saddle, 

into a small inner branch and a large outer branch; 

first lateral saddle bifid, a little smaller than ventral 

saddle, longer than wide; second lateral saddle 

smaller than first lateral saddle, nearly as wide as 

long, and has its inner branch on the umbilical margin. The tubercle is in the outer branch of the second 

lateral saddle.

The holotype is slightly compressed. Another 

specimen (U.S.N.M. 104124) that appears to be undistorted at a diameter of about 9 in. (230 mm) has 

a whorl height of 60 mm., a whorl thickness of 49 

mm., and an umbilical width of about 175 millimeters. The small specimen shown on pi. 17, figs. 3, 

6, has corresponding dimensions of 60 mm., 26 mm., 

20 mm., and 17 mm.

The species is somewhat similar to K. gitinsi McLearn (1929, p. 8, pi. 3, fig. 1, pi. 4, fig. 1, pi. 8, fig. 

5) but has much coarser ribbing, fewer secondaries, 

more prominent tubercles, and more complicated 

sutures, and a longer first lateral lobe. K. plenus 

(McLearn) (1929, p. 5, pis. 1, 2) and K. loganianus 

(Whiteaves) (1876, p. 27, pi. 8, fig. 2) have less 

similar ornamentation and thicker whorl sections. 

The ornamentation is similar to that in Gowericeras, 

but the primaries are shorter, the inner whorls are

much more tightly coiled, the adults attain a much 

larger size, and the suture is somewhat different. It 

has much coarser ribbing and stronger tubercles 

than K. rockymontanus Imlay.

Holotype, U.S.N.M. 104125, paratypes, U.S.N.M. 

104122-104124, 104126-104130.

Ellis group, Rierdon formation, Kepplerites mclearni beds, U.S.G.S. Mes. Iocs. 7124, 10730, 10732, 

18726, 18737, 18738, 18750, 19203, 19211, 19572, 

19583, and 19584.

Kepplerites (Seymourites) rockymontanus Imlay, n. sp. 

Plate 18; plate 19, figures 1, 2

This species is represented in collection? from 

Montana by 2 fairly well preserved specimens and 

by 10 fragments of which most have been crushed 

laterally. The adult whorls before compression were 

probably broadly rounded and wider than higrh. On 

the inner whorls the flanks are fairly flat, round 

abruptly into a low, vertical umbilical wall, and converge slightly above to the nearly flat venter. During 

growth the flanks become more convex and on the 

adult whorls round evenly into a low, gently inclined 

umbilical wall and an evenly rounded venter. The 

umbilicus is fairly narrow on the inner whorls but 

becomes wide on the adult. Most of the widening 

occurs at diameters between 4 and 5 in. On the largest specimen (pi. 18) the body chamber is incomplete, and is represented by slightly more th^n one 

complete volution.

On the inner whorls the ribs are sharp and moderately spaced. The primary ribs curve backward on 

the umbilical wall, curve forward on the lower third 

of the flanks, and terminate in small, acute, radially 

elongated tubercles. The secondary ribs incline 

gently forward, are slightly flexuous, and cross the 

venter transversely. Trifurcation is the common 

mode of branching posterior to the body whorl. 

Most rib branches are separated by single intercalated ribs which begin at or slightly above 1he line 

of tubercles. During growth the ribs gradually become more widely spaced and more elevated but the 

tubercles remain only moderately conspicuous. On 

the adult body chamber the ornamentation changes 

considerably. The tubercles disappear, the ribs become broader and lower, many ribs are single, 

branching occurs only in pairs, and intercalary ribs 

become uncommon.

The suture line is fairly deeply cut. Ventral lobe 

a little longer than wide; first lateral lobe about same 

length as ventral lobe but narrower; second lateral 

lobe shorter arid relatively wider than first lateral 

lobe. Saddles are wider than lobes; ventral saddle
WESTERN INTERIOR UNITED STATES 27

longer than wide and divided by a lobule, which is 

shorter than the saddle, into a small inner branch 

and a large outer branch; first lateral saddle bifid, 

much smaller than ventral saddle, not as long as 

wide; second lateral saddle smaller than first lateral 

saddle, much wider than long, and has its inner 

branch on the umbilical margin. The tubercle is on 

the outer border of the second lateral saddle.

Most of the specimens are so distorted that accurate measurements cannot be made. The largest 

specimen must have had a diameter of 11 in. before 

compression.

This species differs from K. mclearni Imlay (1948, 

p. 24, pi. 8, figs. 1, 2, 5, 7-10, pi. 9, fig. 1) by having 

much weaker ornamentation, and more secondary 

ribs. The evolute coiling of its outer two whorls is 

in marked contrast to that of the described Canadian 

species, although the ribbing on its penultimate 

whorl resembles that in K. gitinsi McLearn (1927, p. 

72, pi. 1, fig. 2; 1929, p. 8; pi. 3, fig. 1; pi. 4, fig. 1: 

pi. 8, fig. 5). Among the somewhat similar species 

from East Greenland, K. rosenkrantzi Spath (1932. 

p. 89, pi. 26, figs, la, b) may be distinguished by its 

weaker ribbing, and K. peramplus Spath (1932, p. 

88, pi. 24, figs, la, b) by its weaker tubercles and 

greater involution.

Times.—Holotype, U.S.N.M. 104131; paratypes, 

U.S.N.M. 104146 a-c.

Occurrence.—Ellis group, Rierdon formation. 

Kepvlerites mclearni beds, U.S.G.S. Mes. Iocs. 10730. 

19203 and 19211, in the Little Rocky Mountains, 

Montana.

Kepplerites (Seymourites) cf. K. rosenkrantzi Spath

Plate 14, figure 9

Two crushed specimens belong to a species whoso 

penultimate whorl bears vigorous ornamentation 

similar to that of K. ingrahami (McLearn) (1929, p. 

9, pi. 7, figs. 1, 2) from British Columbia or that of 

K. rosenkrantzi Spath (1932, pi. 19, fig. 3, pi. 26, 

fig. 1) from East Greenland. Particularly conspicuous are the conical tubercles at the ends of the short, 

prominent primary ribs. From these pass three or 

four sharp, nearly straight secondaries that incline 

slightly forward. Between most rib-bundles are one 

or two intercalated ribs. This ornamentation 

changes completely on the body whorl where tuberculation is lacking and the ribbing consists of striae 

or of bundles of striae. The fading of the ribs is 

similar to that in K. rosenkrantzi but apparently has 

gone one step farther. The manner of coiling is likewise similar to that of K. rosenkrantzi. Comparisons

of the whorl sections cannot be made because of defective preservation.

Figured specimen: U.S.N.M. 108279.

Ellis group, Rierdon formation, Kepplerites mclearni beds, U.S.G.S. Mes. loc. 7124.

Kepplerites (Seymourites) tychonis Ravn 

Plate 20, figure 1-3

Macrocephalites sp. cf. compressus (Quenstedt) Madsen,

Meddelelser om Gr0nland. Bind 29, p. 192, 1904. 

Kepplerites tychonis Ravn, Meddelelser om Gr0nland, Pind

45, p. 490, pi. 37, fig. 1, 1911. 

Kepplerites (Seymourites) tychonis Ravn, Spath, Meddelelser

om Gr0nland, Bind 87, pp. 83-87, pi. 23, fies. 1-3 pi.

24, fig. 6, pi. 25, figs. 1-3, pi. 26, fies. 3. 6, 1932. 

Kepplerites (Seymourites') cf. K. tychonis Ravn. Imlay, U. S.

Geol. Survey Prof. Paper 214-B, p. 25, pi. 8, figs. 3, 4,

6, 1948.

The presence of this distinctive, finely ribbed species of Kepplerites in both Montana and Alaska has 

been confirmed by recent collections. In Montana it 

characterizes beds lying between the zones of Gowericeras subitum and Kewwlerites mclearni. In Alaska it is associated with Paracadoceras and GuHeJmiceras and occurs near beds that have furnished Gowericeras. It appears to be a useful stratigrarhic 

marker for beds equivalent to the lower part of the 

Sigaloceras calloviense zone.

Figured specimens: U.S.N.M. 104172, 104173.

Ellis group. Rierdon formation. Kepplerites tychonis beds, U.S.G.S. Mes. Iocs. 19194, 19629, 20?SO, 

20363, 20390, and 20391.

Kepplerites (Seymourites) landuskiensis Imlay, n. sp. 

Plate 19, figure 3, 6-8

Four specimens of this species are at hand. Of 

these, two are fragments of the penultimate whorl. 

The holotype appears to be nearly undeformed, although one side is much corroded and its inner 

whorls are not preserved. Whorls ovate, higher than 

wide, becoming lower on the anterior third of the 

body whorl. Flanks somewhat flattened, converging 

toward venter, rounding evenly into umbilicus. Venter flattened on penultimate whorl, narrowly arched 

on body whorl. Umbilicus narrow at posterior end 

of penultimate whorl, becoming wide anteriorly: 

umbilical wall low and vertical on penultimate wh^rl, 

becoming gently inclined on body whorl. Body chamber represented by about five-sixths of a whorl. 

Aperture marked by a weak, forwardly inclined 

constriction.

The ribbing on the penultimate whorl is sharp and 

fairly widely spaced. The primary ribs are radial on 

the umbilical wall, curve forward on the lower th ird
28 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

of the flanks, and terminate in small, conical tubercles. From these pass two or three weaker secondary 

ribs that incline forward in a gently flexuous manner 

but cross the venter transversely. Most rib bundles 

are separated by single intercalated ribs. Anteriorly 

on the body whorl the ribs become fairly thick, 

closely-spaced, and more inclined on the flanks, bifurcation becomes the common mode of branching, and 

the point of furcation becomes lower. Tiny, conical 

tubercles persist as far as the aperture. The body 

whorl of the holotype has 47 primary ribs and 146 

secondary ribs.

The suture line is poorly preserved. The holotype 

at a diameter of 140 mm has a whorl height of 50 

mm. At the anterior end the diameter is 161 mm, 

the whorl height 48 mm, and the whorl thickness 

about 39 mm.

Compared with K. rockymontanus Imlay, this species becomes evolute much more quickly, has fewer 

secondary ribs, and more persistent tuberculation. 

Its lateral aspect is similar to that of K. rosenkrantzi 

Spath (1932, p. 89, pi. 19, fig. 3, pi. 26, fig. 1), but it 

is more compressed, has weaker tubercles, and fewer 

secondary ribs. K. peramplus Spath (1932, p. 88, pi. 

24, figs, la, b) is much stouter, has more flexuous 

ribbing, and weaker tubercles.

Holotype, U.S.N.M. 108280; paratype, U.S.N.M. 

108281.

Ellis group, Rierdon formation, Kepplerites mclearni beds, U.S.G.S. Mes. Iocs. 18735 and 18726.

Kepplerites (Seymourites) planiventralis Imlay, n. sp. 

Plate 20, figures 4-9

The three specimens illustrated are the only known 

representatives of the species. Whorl section elliptical, much higher than wide. Flanks flattened, rounding rather abruptly into umbilical wall. Venter 

highly arched, and marked by a persistent narrow, 

flattened area along the mid-ventral line. Umbilicus 

narrow at posterior end of penultimate whorl but 

widening fairly rapidly anteriorly; wall low and vertical on penultimate whorl, becoming steeply inclined 

on body whorl. Body chamber incomplete, represented by at least half a whorl.

The ribbing is sharp, widely spaced and decidedly 

flexuous. The primary ribs are radial on the umbilical wall, curve forward on the lower two-fifths of 

the flanks, and terminate in small, conical tubercles. 

From these on the penultimate whorl pass bundles of 

two or three secondary ribs that tend to curve forward near the middle of the flanks but cross the venter transversely. On the body chamber the furcation 

points become indistinct. Some intercalated ribs

arise at or just above the zone of tuberculation. 

Others occur only on the upper third of the flanks. 

Forward arching of the ribs on the flanks is not 

nearly as conspicuous as on the penultimate whorl.

The suture line is not well preserved.

The holotype at a diameter of 111 mm has a whorl 

height of 47 mm and a whorl thickness of 37 mm. 

At a diameter of 150 mm, the whorl height is 61 mm 

and the whorl thickness 41 mm.

This species most resembles K. landuskiensis of 

any of the associated species in the Rierdon formation. Its ribbing is coarser on the penultimate whorl, 

more widely spaced on the body whorl, and much 

more flexuous on the flanks; its form is mucJ- more 

compressed; and the persistent ventral flattening is 

auite distinctive. It is much more compressed than 

K. rosenkrantzi Spath (1932, p. 89, pi. 19, fig. 3, 

pi. 26, fig. 1), which has similar ribbing.

Holotype. U.S.N.M. 108282; paratypes, U.S.N.M. 

108983, 108284.

Ellis group. Rierdon formation, Kepplerites meleami beds, U.S.G.S. Mes. Iocs. 18754 and 19211.

Genus Gowericeras Buckman 1921

Gowericeras subitum Imlay 

Plate 21, figures 1-18; plate 22, figure 9

Cadoceras subitum, Imlay, U. S. Geol. Survey Prof. Paper 

214-B, pp. 22-24, pi. 7, figs. 1-11, 1948.

This species is represented in collections in hand 

by about 80 specimens and is much more common 

than any of the associated species of Gowericeras.

The inner whorls are subquadrate, higher than 

wide, embrace about four-fifths, have flattened flanks 

that round rather abruptly into a flattened venter, 

and have a low, steep umbilical wall. During growth 

the whorls become subovate, wider than high, thickest near the umbilicus, and considerably more evolute; the flanks become gently convex and round 

evenly into a moderately broad venter; the urrbilicus 

chaneres from fairly narrow to moderately wHe; the 

umbilical wall becomes moderately high, nearly vertical, and rounds rapidly into the flanks. The body 

chamber is represented by about five-sixths of a 

whorl. The aperture is inclined forward in a slightly 

sinuous manner. It is preceded by two weak ribs 

behind which is a broad shallow constriction that is 

not evident on the venter (pi. 21, figs. 11, 12).

The character of the ribbing changes fairly abruptly several times during growth following- a constriction and then an abrupt thickening of the whorl 

section, but. the greatest change occurs at the beginning of the adult body chamber. The smallest whorls 

(pi. 21, figs. 5, 8, 9) up to a diameter of about 30
WESTERN INTERIOR UNITED STATES 29

mm have' moderately strong primary ribs that are 

widely spaced, incline backward slightly on the umbilical wall, incline forward on the lower third of the 

flank, and terminate abruptly in weak, radially elongated tubercles. From the tubercles arise pairs of 

secondary ribs that are considerably weaker than the 

primary ribs. Between successive rib pairs are two 

or three intercalary ribs that begin between the tubercles and the middle of the flank and are as strong 

as the paired ribs. The paired and intercalary ribs 

incline forward on the flanks and cross the venter 

transversely or with a slight forward bend.

At a diameter of about 30 mm the shell has a 

rather pronounced constriction that is followed by 

an abrupt thickening of the whorl section (pi. 21, 

figs. 2, 3). Several other constrictions are generally 

present in the next three-fourths whorl up to a diameter of about 45 mm. The thickening is accompanied 

by coarsening of the primary ribs which become relatively much stronger than the secondaries. The primaries terminate in weak radially elongate tubercles 

from which pass two or three secondaries. Intercalary ribs become less common anteriorly and generally only one is present between successive groups 

of branched ribs. The forward inclination of the 

ribs is very marked, and the secondaries bend forward gently on the venter.

On the body chamber the primary ribs abruptly 

become much stronger and more widely spaced. The 

weak radially elongate tubercles persist, but branching occurs well above the tubercles at or above the 

middle of the flanks. For most primary ribs there 

are generally three secondaries, of which two are 

connected with the primary and the other begins 

high on the flank. Near the aperture there are only 

two secondaries for each primary. All secondary 

ribs bend forward considerably on the venter and 

are slightly weaker there than on the flanks.

The suture line (pi. 21, fig. 10) is rather simple. 

Ventral lobe longer than wide; first lateral lobe trifid, 

slightly longer than wide, a little shorter than ventral lobe; second lateral lobe trifid, much smaller 

than first lateral lobe. Saddles wider than lobes; 

ventral saddle bifid with largest branch on outer 

side, as long as wide; first lateral saddle broad and 

bifid, much smaller than ventral saddle. The tubercle 

is in the outer branch of the first lateral saddle.

This species is characterized by the abrupt coarsening of the ribbing at the beginning of the body 

chamber and by the ribs becoming widely spaced on 

the anterior half of the body whorl.

Dimensions in millimeters and in ratios are as 

follows:

Specimen

Paratype (pi. 21, figs.

8,9).

Paratype (pi. 21, figs.

2,3).

Holotype (pi. 21, figs.

13, 14).

Paratype (pi. 21. fig.

4).

Paratype (pi. 21, figs.

17, 18).

Diameter

25

42

56

61

71

Whorl 

height

11

19

24

23

26

.5 (0.46)

. (0.45)

. (0.43)

. (0.38)

. (0.37)

Whorl 

thickness

8

20

25

26

.5 (0.34)

. (0.48)

. (0.45)

?

. (0.37)

Umbir^al 

width

6 (0.24)

11 (0.26)

16 (0,29)

19 (0.31)

24 (0.34)

Holotype, U.S.N.M. 104155; paratypes, U.S.N.M. 

104153, 104154, 104156a-c; plesiotypes, U.S.N.M. 

108285a-c, 108286, 108287a, b.

Ellis group, Kierdon formation, Gowericeras subitum beds, U.S.G.S. Mes. Iocs. 19169, 19175, 19179- 

19182,19602, 19616, and 20365.

Gowericeras subitum var. dlstinctum Imlay, n. var. 

Plate 22, figures 5, 6

One adult specimen having a well-preserved aperture differs from the typical form of G. subitum by 

the more gradual coarsening of the ribbing on the 

body whorl, closer spacing of the primary ribs, and 

fewer secondary ribs for each primary. The ribbing 

on the body whorl is much weaker and denser than 

in Goioericeras costihians Imlay. Gowericeras ap~ 

proximatum (Buckman) (1922, pi. 336) has similar 

ornamentation and shape, but bears distinct ventral 

tubercles bordering a flattened mid-ventral area.

Holotype, U.S.N.M. 108288.

Ellis group, Kierdon formation, Gowericeras subitum beds, U.S.G.S. Mes. Iocs. 19179.

Gowericeras costihians Imlay, n. sp. 

Plate 22, figures 1-4, 7, 8

This species is represented by 8 specimens that 

show only the outer two whorls. Whorls subov.te, 

wider than high, embracing about three-fourths. 

Anterior part of body whorl contracting somewhat 

from remainder of shell. Flanks gently convex. Venter broadly rounded. Umbilicus changes from fairly 

narrow to moderately wide; wall vertical at b^se, 

rounds rapidly into flanks. Body chamber incomplete, represented by nearly three-fourths of a whorl.

The ribbing is strong, rather widely spaced, and 

coarsens gradually on the outer two whorls. The 

primary ribs are prominent, widely separated, nearly radial, and terminate at the top of the lower twofifths of the flanks in small, acute tubercles. From 

these pass two slightly weaker secondary ribs that
30 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

incline forward on the flanks, arch forward gently 

on the venter, and are not reduced in strength along 

the mid-ventral line. The pairs of forked ribs are 

separated by single short ribs which pass across tne 

venter into paired ribs.

The first lateral lobe is slightly longer than the 

ventral lobe. The second lateral lobe is much shorter 

than the first. The saddles are broader than the 

lobes. The tubercle is in the inner branch of the first 

lateral saddle.

The specimens are either too distorted or too incomplete lor accurate measurements.

This species bears considerable resemblance to G. 

subitum imiay. It differs by having a thicker whorl 

section and stronger, sharper, more widely-spaced 

ribbing that is particularly stronger on the venter of 

the adult body chamber.

Holotype, U.S.N.M. 108289; paratypes, U.S.N.M. 

108^90, 108314.

Eliis group, Rierdon formation, Gowericeras subitum beds, U.S.G.JS. Mes. Iocs. 19169, 19179, 19180, 

and 19182.

Gowericeras costimedium Imlay, n. sp. 

Plate 23, figures 14-16

This species is represented by three specimens, of 

which two are fragments of the body whorl. Shell 

stout for genus. Whorls semicircular in section, 

wider than high, embracing about three-fifths. 

Flanks and venter evenly rounded. Umbilicus moderately wide, widening rapidly near end of body 

whorl; wall fairly low, steeply inclined, rounding 

evenly into flanks. Body chamber incomplete but 

represented by at least three-fourths of a whorl.

The ribs on the body whorl are low, moderately 

spaced, incline forward slightly on the flanks, and 

cross the venter transversely. The primary ribs are 

nearly twice as strong as the secondary ribs and 

terminate in radially elongate tubercles at about the 

top of the lower two-fifths of the flanks. From these 

tubercles pass three or four secondary ribs. Single, 

short ribs are intercalated between most rib bundles. 

The body whorl bears 23 primary ribs and about 80 

secondary ribs.

The suture line is characterized by broad saddles 

and narrow lobes. The first lateral lobe is about the 

same length as the ventral lobe. The tubercle is in 

the outer branch of the first lateral saddle.

The holotype at a diameter of 64 mm has a whorl 

height of 25 mm, a whorl thickness of 28.5 mm, and 

an umbilical width of 24 mm.

This species differs from G. subitum and its varieties by having weaker, denser ribbing that does not

coarsen rapidly on the body whorl. Its ribbing is 

similar to that on G. planum Buckman (1922, pi. 

287), although probably not as dense. In this respect 

it is intermediate between G. planum, and G. metorchum Buckman (1921, pi. 254).

Holotype, U.S.N.M. 108291.

Ellis group, Rierdon formation, Gowericeras beds, 

U.S.G.S. Mes. Iocs. 19179, 19180, 19585.

Gowericeras costicrassum Imlay, n. sp. 

Plate 23, figures 1-12, 18

This species is represented by 10 specimens. Shell 

very stout, coronate. Whorls semicircular in section, 

much wider than high, embracing about three-fifths. 

Flanks convex. Venter broadly rounded, faintly flattened at diameters less than 20 mm. Umbilicus moderately narrow, widening slightly at anterior end of 

body whorl; wall high, vertical at base, steeply inclined above and rounding rather abruptly into 

flanks. Body chamber represented by about fourfifths of a whorl. Aperture inclined forward, abruptly truncated, preceded by a single, stronjr rib.

The ribbing is vigorous and the lateral spines are 

prominent. The primary ribs are high and narrow, 

trend radially on the umbilical wall, and curve forward on the lower two-fifths of the flanks whe^e they 

terminate in acute tubercles. From the tubercles 

pass two- or rarely three, narrow but weaker secondary ribs that incline forward on the flanks and 

arch forward gently on the venter. Most sets of 

branched ribs are separated by single, short ribs. A 

marked diminution in strength of the secondary ribbing occurs on half a whorl between diameters of 

about 22 to 27 mm. This diminution does not affect 

the primary ribs or the tubercles and is not related 

to changes in whorl shape. Anteriorly all ornamentation becomes more prominent and more widely 

spaced. The small specimen shown on pi. 23, figs. 

5-7, has 78 secondary ribs and 21 primary riH, The 

much larger holotype has 68 secondary ribs and 21 

primary ribs.

The suture line has broad saddles and narrow 

lobes. The first lateral lobe is shorter than the ventral lobe. The tubercle is in the inner branch of the 

first lateral saddle.

This species is characterized by its stout form and 

coarse ornamentation. It is more coronate than any 

described species of Gowericeras.

Holotype, U.S.N.M. 108294; paratypes, U.S.N.M. 

108292, 108293a, b, 108295a, b.

Ellis group, Rierdon formation, Gowericeris sub-
WESTERN INTERIOR UNITED STATES 31

itum beds, U.S.G.S. Mes. Iocs. 19169, 19170, 19180, 

19182, and 19602.

Dimensions in millimeters and in ratios are as 

follows:

Specimen

Paratype (pi. 23, figs.

12, 18).

Paratype (pi. 23, figs.

10, 11).

Paratype (pi. 23, figs.

5,7).

Paratype (pi. 23, fig.

4).

Holotype (pi. 23, figs.

1,3).

Diameter

21.5

26

35

43

71

Whorl 

height

9 (0.42)

12 (0.46)

17 (0.48)

19 (0.44)

28 (0.39)

~

Whorl

thickness

11 (0.51)

18 (0.69)

21 (0.60)

26 (0.60)

41 (0.57)

Umbilical 

width

6? (0.28)

7 (0.27)

9 (0.26)

12 (0.28)

23 (0.32)

Gowericeras costidensum Imlay, n. sp. 

Plate 22, figures 10-13

Seven specimens of this species are available. 

These show only the body whorl. Shell fairly stout. 

Whorls semicircular in section, wider than high, 

embracing preceding whorl about three-fifths. 

Flanks evenly convex. Venter broadly convex. Umbilicus moderately narrow; wall vertical at base, 

rounding evenly into flanks. Body chamber represented by three-fourths of a whorl. Aperture inclined forward, abruptly truncated, preceded by two 

weak single ribs.

The ribbing is fine and dense. The primary ribs 

are narrow, much stronger than the secondary ribs, 

trend backward on the umbilical wall, and curve forward on the lower two-fifths of the flanks, where 

they terminate in tiny, acute tubercles. From the 

tubercles pass bundles of 3 or 4 thread-like secondary 

ribs, between which lie 2 or 3 short ribs that begin 

a little higher than the zone of tuberculation. The 

secondary ribs incline forward on the flanks and 

arch forward gently on the venter. They are so fine 

that despite their large number they are separated 

by interspaces from 2 to 3 times their own width. 

Near the anterior end of the holotype there are about 

25 secondary ribs in 25 mm and on the paratype at 

a comparable position about 21 ribs in 25 mm.

The suture line is poorly preserved and cannot be 

traced.

The holotype is too distorted for accurate measurements. The paratype at a diameter of 53 mm has a 

whorl height of 23 mm, a whorl thickness of 29 mm, 

and an umbilical width of 16 mm.

The species is characterized by its fine, dense ribbing and its fairly stout form. Among European

species, Goivericeras toricellii (Oppel) (in Buekrran, 

1922, pi. 292) has nearly as dense ribbing but it is 

not as stout and its venter is flattened. None of the 

species of Gowericeras from Alaska or the western 

interior of the United States has comparable ribbing.

Holotype, U.S.N.M. 108297; paratype, U.S.N.M. 

108296.

Ellis group, Rierdon formation, at base of Gowericeras beds, U.S.G.S. Mes. Iocs. 18718, 19170, 

19212,19576 and 19585. This species has been found 

associated with Cadoceras shoshonense Imlay but not 

with Gowericeras subitum Imlay. In the Little Rc^.ky 

Mountains it was found at Chalk Butte dome only 

1 foot above a specimen of Arcticoceras codyense 

Imlay and at Camp Creek dome only 5 ft. alove 

Arcticoceras codyense. In the Big Snowy Mountains 

it was collected within 15 ft. of the base of the 

Rierdon formation. In the Sawtooth Range south of 

Glacier Park, it was obtained near the top of the 

Rierdon formation directly below beds contairing 

Gowericeras subitum and directly above beds containing Arcticoceras.

Genus Cosmoceras Waagen 1869

Subgenus Gulielmiceras Buckman 1920

Cosmoceras (Gulielmiceras) knechteli Imlay, n. sp.

Plate 5, figures 1-5

This species is represented by 16 specimens. 

Whorls elliptical, much higher than wide, embracing 

about two-thirds. Flanks flattened below, converging slightly above, rounding abruptly into the umbilical wall. Venter narrow, truncated, slightly concave. Umbilicus fairly narrow, shallow, not enlarging on body whorl; wall low and vertical. Body 

chamber represented by three-fourths of a whorl. 

Aperture not perfectly preserved, but the posterior 

end of the lateral lappet is present on several specimens.

The ribbing is dense, fine, and flexuous. The primary ribs are narrow and a little higher than the 

secondary ribs. They trend backward on the ujner 

part of the umbilical wall and trend forward on the 

lower third of the flanks where they terminate in 

small tubercles that are elongate radially. The primary ribs are slightly swollen on the umbilical e^ge 

without forming true tubercles. The secondary ribs 

bend forward near the middle of the flanks and then 

recurve slightly and cross the venter transversely. 

Near the lateral lappet the forward bending becomes 

pronounced. Bifurcation is the common mode of rib 

branching and most pairs of forked ribs are separated by single ribs that begin along the line of 

furcation. A few intercalated ribs arise higher on
32 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

the flanks. Where the secondary ribs cross the 

ventro-lateral shoulders they give rise to tongueshaped tubercles that are steepest on their anterior 

sides. The body whorl of the holotype has 42 primary 

ribs and 113 secondary ribs.

The suture line cannot be traced.

The holotype has been crushed laterally so that 

measurements are useless. The paratype shown on 

plate 5, figures 1 and 2, at a diameter of about 35 

mm, has a whorl height of 17 mm a whorl thickness 

of 10.5 mm, and an umbilical width of 7.5 mm.

This species is characterized by its dense ribbing 

and compressed shape. Its ribbing is similar to that 

of a Gulielmiceras from England figured by Brinkman (1929, pi. 1, fig. 1), but it differs by having a 

ribbed venter and by lacking umbilical tubercles. 

C. (G.) pauper Spath (1932, p. 96, pi. 24, figs. 3a, b; 

pi. 26, fig. 5) has weaker ribbing and is less compressed.

This species is named in honor of Maxwell M. 

Knechtel of the Geological Survey who spent several 

summers studying the geology of the Little Rocky 

Mountains.

Holotype, U.S.N.M. 104152; paratypes, U.S.N.M. 

104151 and 108298.

Ellis group, Rierdon formation, Kepperites mclearni beds, U.S.G.S. Mes. localities 10730, 10732, 

11973, 18726, 18737, 19203, 19211, 19584.

Cosmoceras (Gulielmiceras) zortmanense Imlay, n. sp. 

Plate 14, figures 1-6, 8

This species is .represented by 18 specimens, of 

which most are fragmentary. Whorls subovate, a 

little higher than wide, embracing about two-thirds. 

Flanks rather flattened in their lower parts, converging gently in upper parts, rounding abruptly into 

umbilical wall. Venter narrow, truncated, slightly 

concave. Umbilicus fairly narrow, shallow, not enlarging on body whorl; wall low and vertical. Body 

chamber represented by three-fifths of a whorl. Posterior end of lateral lappet preserved on five specimens.

The ribbing is fine, fairly closely spaced, and moderately flexuous. The primary ribs are high and narrow, trend nearly radially on the umbilical wall, 

curve forward on the lower third of the flanks, and 

terminate in weak radially elongate tubercles. From 

these pass pairs of secondary ribs that bend forward 

near the middle of the flanks but cross the venter 

transversely. Between most of the pairs are single 

ribs that begin at or just above the zone of furcation. 

Near the lateral lappet the secondary ribs bulge forward considerably. On the ventro-lateral angles the

ribs are produced in tongue-shaped tubercles that 

are steepest on their anterior sides.

The suture line is poorly preserved. The paratype 

shown on pi. 14, figs. 3, 4, at a diameter of 35 mm 

has a whorl height of 13.5 mm, a whorl thichness of 

9 mm, and an umbilical width of 10 mm.

This species differs from C. (Gulielmiceras) 

knechteli Imlay in its sparser, coarser ribbing, lower 

whorl section, broader venter, and slightly wider 

umbilicus. Its ribbing is nearly the same as in a 

species occurring in the lower part of the Chinitna 

siltstone in Alaska, but it is more compressed1 and its 

venter narrower.

Holotype, U.S.N.M. 108299; paratypes, U.S.N.M. 

108300-108302.

Ellis group, Rierdon formation, Kepplerites mclearni beds, U.S.G.S. Mes. Iocs. 10729, 18735, 18742, 

and 18750.

Cosmoceras (Gulielmiceras) vigorosum Imlay, n. sp. 

Plate 9, figures 4, 5, 9

One nearly complete adult specimen has much 

coarser ribbing than any of the associated forms of 

Gulielmiceras. Whorls elliptical, much higher than 

wide, embracing about two-thirds. Flanks fattened 

in lower two-thirds, converging slightly above into a 

narrow, truncated venter. Umbilicus fairly narrow, 

shallow, not enlarging on body whorl; wall low and 

vertical. Body chamber occupies about three-fourths 

of a whorl. Aperture not perfectly preserved, but 

there are remnants of a lateral lappet extending 

about 6 mm beyond the most anterior rib.

The ribbing is prominent and fairly widely spaced. 

The primary ribs curve forward on the lower third 

of the flanks and terminate in small radially elongate 

tubercles. From these pass pairs of secondary ribs, 

of which one rib is commonly indistinctly connected. 

In addition, most rib pairs are separated by single 

intercalated ribs that begin along the zone of furcation. All the secondary ribs bend forward r^ar the 

middle of the flanks, then recurve slightly, curve 

forward again near the venter, and cross the venter 

transversely. The forward bending becomes accentuated near the aperture. Small tongue-shaped tubercles arise from the ribs along the ventrolateral 

shoulders.

The suture line is poorly preserved.

The holotype at a diameter of 46 mm has a whorl 

height of 19 mm, a whorl thickness of 11 mm, and 

an umbilical width of 13 mm. The specirr^n has 

been slightly crushed laterally.

The species has coarser ribbing than C. (G: .) zortmanense Imlay. Compared with C. (G.) stutchburii
WESTERN INTERIOR UNITED STATES 33

(Pratt) (Buckman, 1924, pi. 531), from England, 

its ribbing is less sharp, its involution is greater, 

and it lacks umbilical tubercles. It is finer ribbed 

and less tuberculate than a species from Russia described by Nikitin (1881a, p. 70, pi. 4, fig. 31), but 

resembles it in the absence of umbilical tubercles. Its 

shape, and its ribbing are similar to that of Sigaloceras enodatum of Lahusen (1883, p. 55, pi. 6, fig. 

9), which does not appear to be the same species as 

described by Nikitin (1881b, p. 112, pi. 3, figs. 12, 

13).

Holotype, U.S.N.M. 108303.

Ellis group, Rierdon formation, Kepplerites 

mclearni beds, U.S.G.S. Mes. loc. 18754.

Subgenus Gulielmites Buckman 1923 

Cosmoceras (Gulielmites) cf. C. obductum (Buckman)

Plate 8, figures 3-5

Two fragments have weak umbilical, lateral and 

ventral tubercles and fine, flexuous, ribbing that is 

indistinct on the lower part of the flanks but sharp 

on the ventral region. The venter is distinctly flattened. The ornamentation closely resembles that of 

C. (Gulielmites) obductum (Buckman) (1925, pi. 

559) and is a little coarser than that of C. (Gulielmites) delicatus (Buckman) (1924, pi. 521). 

Figured specimens, U.S.N.M. 108304, 108305. 

Ellis group, Rierdon formation, Kepplerites 

mclearni beds, U.S.G.S. Mes. Iocs. 10729, 18735.

Cosmoceras (Gulielmites) cf. C. Jason (Reinecke) 

Plate 8, figures 1, 6, 8

One small fragment has three rows of tubercles, 

flexuous ribbing, indistinct furcation, and continuous ribbing across a flattened venter. The ventral 

tubercles are larger than the others, and elongated 

spirally. In several places two ribs unite in a ventral tubercle. The umbilicus is narrow.

The ribbing is coarser than that of most species 

of Gulielmites, but is not much coarser than some 

specimens from France assigned to Cosmoceras 

(Gulielmites) jason (Reinecke) by Douville (1915, 

pi. 10, figs. 6-8), or from Russia assigned to C. jason 

by D'Orbigny (1845, p. 442, pi. 36, figs. 9-15). The 

narrow umbilicus is more similar to that of the 

Russian than of the French specimens.

Figured specimen, U.S.N.M. 108306.

Ellis group, Rierdon formation, Kepplerites 

mclearni beds, U.S.G.S. Mes. loc. 19196.

Genus Frocerites Siemiradzki 1898

Procerites spp.

Plate 23, figures 13, 17; plate 24, figures 9-11 

A few specimens of Procerites have been found in

the Arcticoceras and Gowericeras beds of the lower 

part of the Rierdon formation in central and western 

Montana. Most of them are poorly preserved sep+ate 

fragments that indicate adults ranging from 8 to 12 

in. in diameter. Their preservation is generally too 

poor for illustration. Four fragments from U.S.G.S. 

Mes. loc. 18753 in the Little Rocky Mountains show 

some resemblance to Procerites funatus (Oppel) in 

Neumayr (1871, p. 40, pi. 14, figs, la, b), but have 

wider spaced ribs. Some specimens from U.S.G.S. 

Mes. localities 18718, 19597, and 19600 in the Sawtooth Range in Montana have rather weak ribbing 

similar to that of Procerites tmetolobus Buckraan 

(1923, pi. 416). The specimens herein figured are 

from Mesozoic localities 18718 and 19603 in the 

Sawtooth Range and may be compared with P. 

moorei (Oppel) in Neumayr (1871, p. 39, pi. 13, 

figs, la, b, c). They differ in having weaker primary 

ribs and forwardly inclined secondary ribs.

Figured specimens, U.S.N.M. 108307, 108308.

Ellis group, Rierdon formation, Arcticoceras b°.ds, 

U.S.G.S. Mes. localities 18718, 18753, 19597, and 

19600; Gowericeras subitum beds at Mes. locality 

19603.

Genus Grossouvria Siemiradzki 1898

Grossouvria warmdomensis Imlay, n. sp.

Plate 19, figures 4, 5

This species is represented by the holotype and by 

a small fragment of a septate whorl that is a little 

larger than the holotype. Whorl ovate in section, 

slightly wider than high, rather evolute; flanks 

gently convex, rounding gradually into umbilical 

wall and evenly arched venter. Umbilicus moderately narrow and deep; wall vertical at base, rounded 

above. Body chamber unknown.

The ribs are characterized by their width anc1 by 

curving backward along the ventral margins. They 

trend radially on the umbilical wall, incline strorgly 

forward on the flanks, recurve abruptly on the margins of the venter, and cross the venter transversely 

or with a slight backward arch. They are not reduced in strength along the venter. The primary 

ribs are fine on the umbilical wall, much broader 

than the interspaces on the flanks, and nearly half 

of them divide into two slightly weaker secondary 

ribs high on the flanks. Most pairs of forked ribs are 

separated by one unforked rib, but some are separated by two. Invariably a rib that is single on one 

flank passes across the venter into a branch cf a 

forked rib.

The suture line is not preserved.

The holotype at a diameter of 43 mm has a w?orl
34 CALLOVIAN (JURASSIC) AMMONITES FROM THE UNITED STATES AND ALASKA

height of 17 mm, a whorl thickness of 16 mm, and 

an umbilical width of 13 mm. At a diameter of 35 

mm the whorl width slightly exceeds the height. The 

specimen has been compressed slightly, mostly at its 

anterior end.

In lateral views the ribbing of this species resembles that of G. retrocostatus (Petitclerc) as figured by Corroy (1932, p. 148, pi. 20, figs. 9, 10), 

but the ribs are not interrupted along the midventral line and the coiling is less evolute. Most 

species of Grossouvria have narrower and more 

widely spaced ribs.

Holotype, U.S.N.M. 108309.

Ellis group, Rierdon formation, Gowericeras costidensum beds, U.S.G.S. Mes. loc. 19585.

Grossouvria spp. 

Plate 24, figures 1-8

One weathered specimen (pi. 24, fig. 7) from the 

Gowericeras subitum zone in the Sawtooth Range 

of Montana differs from G. warmdomensis Imlay in 

its finer and more widely spaced ribbing and is possibly only a variety of that species. It appears to 

be more finely ribbed than most of the described 

species from Europe.

Several fragments (pi. 24, figs. 1, 3, 8) from the 

Kepplerites mclearni zone in the Sweetgrass Hills

may be compared with G. calloviense (Loczy) (1915, 

p. 380, pi. 22 (10), figs. 1, 2) from Hungary. The 

backward curvature of the ventral ribbing is conspicuous and the ribs are reduced in strength along 

the mid-ventral line.

Three specimens (pi. 24, figs. 2, 4-6) from the 

Gowericeras subitum zone in the Big Snowy Mountains belong to a species that differs from G. warmdomensis Imlay in having a subquadrate whorl section, flattened flanks, lower points of rib branching, 

and finer ribbing. It is more involute than most 

species of Grossouvria but resembles a species described by Neumayr (1871, p. 34, pi. 12, figs, 4a, b) 

as Perisphinctes aurigerus Oppel. Likewise, its ribbing is weaker on the lower parts of the flanks than 

in most species of Grossouvria. However, a generic 

assignment to Grossouvria, or possibly to Siemiradzkia is indicated by the backward curvature of the 

secondary ribs and by the presence of many single 

ribs.

Figured specimens, U.S.N.M. 108310a-c, 708311- 

108313.

Ellis group, Rierdon formation, Gowericeras 

subitum beds at U.S.G.S. Mes. localities 19180, 

19182, and 20365, and Kepplerites mclearni beds at 

Mes. localities 21467.

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WESTERN INTERIOR UNITED STATES 35

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7 pis.

———— 1881b-1885a, Der Jura der Umgegend von Elatma; 

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(1885), pp. 43-66, pis. 7-11 (IX-XIII) : Soc. Imp. Natur. 

Moscou, Nouv. Mem., vols. 14, 15.

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INDEX

[Italic numbers indicated descriptions]

Page 

A

Ammonite associations ................................... 11

distribution ......................................... 11

angustiumbilicata, Xenocephalites parvus ............... 19

approximatum, Gowericeras ............................. 29

Arctocephalites ..................................... 2, 4, 5, 21

Aretieoceras ............... 1, 2, 3, 5, 7,11,12,17,18, 23,25, 33

codyense ................. .2,5,6,7,17,20,21, 22, 31, pi. 4

codyense zone ....................................... 5-6

crassicostatum ......................... 2, 5, 6, 20, 21, pi. 3

henryi ...........................2, 5, 6, 7, 20, 21, 28, pi. 5

ishmae .......................................2,20, 21, 22

fcoe/w ..............................................2, 5, 20

foveamim .................................2, 6, 7, #i, pi. 3

michaelis ............................................ 2

rierdonense .......................2, 6, 20, 21, 22, pis. 2, 3

w. sp. .............................................SO, pi. 3

B

bearpawensis, Xenocephalites ......................6, iP, pi. 1

Belemnites .............................................. 10

Belt Island .............................................. 10

Big Horn Mountains ..................................... 9

Biologic analysis of ammonite fauna..................... 1

Bison Basin area ........................................ 9

Black Hills .............................................. 9

borealis, Xenocephalites ................................. 19

Brachiopods ............................................. 10

Cadoceras ...........................1, 2, 7, 8,11,12,17,18, 23

calyx ................................................ 5

elatmae ............................................. 5

frandscus ....................................... 3, 24, 25

freboldi .............................................3,24

muelleri ......................... 3, 6, . .23, 24, 25, pis. 7,9

piperense .............................. 3, 6, 25, pis. 12-14

shoshonense .......................... 3, 6, ##, 24, 31, pi. 6

stenolobum .......................................... 2

subitum ............................................. 28

subpatruum ...................................... 2, 22,23

surense ............................................. 3, 22

tetonense ..............................3, 6, 24, pis. 10, 11

calloviense, Sigaloceras .....................2, 3, 4, 7, 8,17, 27

calyx, Cadoceras ......................................... 5

Camptonectes ............................................ 10

Canyon Springs sandstone member....................... 4, 9

Cardioceratidae .......................................... 1,2

Chamoussetia ......................... <.................. 2, 5

codyense, Aretieoceras .........2, 5, 6, 7,17, 20, 2i, 22, 23, pi. 4

Comparison with other faunas........................... 8

compressus, Macrocephalites ............................. 27

Conglomerate ............................................ 9

coronatum, Erymnoceras ...............................3, 7, 8

Correlation of f aunal zones .............................. 5-8

Cosmoceras .............................................. J, 17

jason ...........................................2, 3, f, 18

obductum ........................................... 4

(Gulielmiceras) ............................... 3, 8,11,17

knechteli ..................................6, «, pi.5

stutehburii ..................................... 32

vigorosum ................................. 6, 5#, pi. 9

zortmanense ............................. 6, 5#, p1 .14

(Gulielmites) .....................................3, F, 11

delicatus ........................................ 33

jason. ...................................6, 8, «, pL 8

obductum ................................. 6, 55, pi. 8

Cosmoceratidae .......................................... 1,2

costicrassum, Gowericeras ........................ 6,50, p1 ,, 23

costidensum, Gowericeras ..............3, 6, 7,17, 23,5i, pJ,. 22

costihians, Gowericeras ...........................6, 29,^,22

costimedium, Gowericeras ......................... 6,50, p?,- 23

Cranocephalites ......................................... 19

crassicostatum, Aretieoceras ............... 2, 5, 6, £0, 21, pi. 3

crinoids .................................................. 10

D

delicatus, Cosmoceras (Gulielmites) ..................... 33

distinctum, Gowericeras subitum .................. 6, #0, pi. 22

E

East Butte, Sweetgrass Hills............................. 9

Ecology .................................................. 8-11

elatmae, Cadoceras ...................................... 5

enodatum, Sigaloceras ................................... 33

Erymnoceras coronatum ................................ 3, 7, 8

Faunal zones ............................................ 5-8

Foraminifera ............................................ 10

franciscus, Cadoceras ................................3,24. 25

freboldi, Cadoceras ......................................E. 24

funatus, Procerites ...................................... 33

6

Geographic distribution .............................tab. 2,14

gitinsi, Kepplerites ....................................26,. 27

Glacier Park area ....................................... 9

Glauconite ............................................... 11

37
38 INDEX

Page

G—Continued

Gowerioeras ..................... 1, 3, 5, 8,11,12,14,17,18, 31

approximatum ...................................... 29

costicrassum .................................. 6, 30, pi. 23

costidensum ................... 3, 6, 7,17, 28, 81, 34, pi. 22

costihians .................................... 6, 29, pi. 22

costimedium .................................. 6, #0, pi. 23

metorchum .......................................... 30

p&mwm ............................................. 30

subitum .........3, 6, 7,11,17, 25, 27, 2S, 29, 30, 31, 33, 34,

pis. 21, 22 

distinctum .................................... 6, 29, pi. 22

zone ............................................ 6,7

toricellii ............................................. 31

Grossouvria, ................................. 4, 8,11,12,17,18

retrocostatiis ........................................ 34

wa-rmdomensis ............................ 6, #3, 34, pi. 19

spp ...........................................6, 54, pi. 24

Gulielmiceras ....................................... 3, 4, 8,18

(Gulielmiceras), Cosmoceras ...................... 3, 8,11, 17

knechteli, Cosmoceras ..........................6, 5i, pi. 5

zortmanense, Cosmoceras ..................... 6, 52, pi. 14

Gulielmites ............................................... 4

(Gulielmites), Cosmoceras ............................3, 8,11

delicatus, Cosmoceras ............................... 33

jason, Cosmoceras ........................... 6, 8, 55, pi. 8

obductum, Cosmoceras ......................... 6, 55, pi. 8

stutchburii, Cosmoceras ............................. 32

vigorosum, Cosmoceras ........................ 6, 52, pi. 9

Gryphaea ................................................ 10

impressimarginata .................................. 5

nebrascensis ........................................ 7,8

Gypsum ................................................. 10

H

Hartville Uplift ......................................... 4

henryi, Arcticoceras .................. 2, 5, 6, 7, 20, 21, 22, pi. 5

Hulett sandstone ........................................ 4,10

impressimarginata, Gryphaea ........................... 5

ingrahami, Kepplerites .................................. 27

ishmae, Arcticoceras .............................. 2, 20, 21,22

jason, Cosmoceras ................................... 2, 3, 4, 18

Cosmoceras (Gulielmites) .................... 6,8, 55, pi. 8

K

Kamptokephalites ....................................... 8

Kepplerites ...................................... 1, 3, 8,11,12

gitinsi ............................................. 26, 27

ingrahami ........................................... 27

landuskiensis ........................................ 28

loganianus .......................................... 26

mclearni ............ 2, 3, 4,11,14,17,18, 26, 27, 52, 33, 34

zone ............................................ 6-8

peramplus ......................................... 27, 28

plenus ............................................... 26

K—Continued

rockymontanus .................................. 3, 26, 28

rosenkrantzi ....................................... 27, 28

tychonis ................................ 3, 7,11,12,17, 27

zone ............................................ 6, 7

(Seymourites) ...................................... 3

landuskiensis ............................ 6, 27, pi. 19

mclearni .............................6,25, pis. 15-17

planiventralis ............................ 6, 2S, pi. 20

rockymontanus ...................... 6, 20, p1 s. 18,19

rosenkrantzi ............................. 6, 27, pi. 14

tychonis ..................................6, 27, pi. 20

Kheraiceras ....................................3, 7, 32,18, 19

knechteli, Cosmoceras (Gulielmiceras) .............6. 5i, pi. 5

kochi, Arcticoceras ....................................2, 5, 20

koenigi, Proplanulites ..........................3, 4, f. 7, 8,17

krylowi, Pleurocephalites ................................ 5

landuskiensis, Kepplerites ............................... 28

Kepplerites (Seymourites) ................... 6, 27, pi. 19

lilloetensis, Lilloettia .................................... 18

Lilloettia .................................2, 7, 8,11,12,17, 18

lilloetensis .......................................... 18

sp .............................................6, iS, pi. 1

Lingula .................................................. 10

Little Rocky Mountains .................................. 10

Localities, descriptions .................................. 14-17

Longaeviceras ........................................... 2

loganianus, Kepplerites .................................. 26

Zoveafmm, Arcticoceras ........................2, 6, 7, 2?, pi. 3

M

mclearni, Kepplerites ... .2, 3, 4,11,14,17,18, 26, 27, 32,33, 34

Kepplerites (Seymourites) .............. .6, 25, pis. 15-17

Macrocephalites ....................................... 5, 8, 23

compressus .......................................... 27

macrocephahis ..........'........................... 3, 5, 8

Macrocephalitidae ....................................... 1, 2

macrocephalus, Macrocephalites ........................ 3, 5, 8

Marine siltstone ......................................... 10

Material available for study............................. 1-2

metorchum, Gowericeras .-. ............................... 30

michaelis, Arcticoceras ................................... 2

Mollusks ................................................. 10

muelleri, Cadoceras .................... 3, 6, 25, 24, 25, pis. 7, 9

Mytilus .................................................. 11

N

nebrascensis, Gryphaea .................................. 7, 8

nikitini, Xenocephalites ............................ 6 10, pi. 1

Nugget sandstone ....................................... 4,9

0

obductum, Cosmoceras ................................... 4

Cosmoceras (Gulielmites) .....................6 55, pi. 8

Oppelia (Oxycerites) .................................... 3

Ostracodes ............................................... 10

Ostrea ................................................... 11

(Oxycerites), Oppelia ................................... 3
INDEX 

Page

39 

Page

S—Continued

Paleogeography, lower Callovian, map................... 13 shoshonense, Cadoceras ................... 3, 6, 23, 24,31, pi. 6

Paracadoceras .................................. 7, 8,12,17,18 Siemiradzkia ............................................ 34

parvus angustiumbilicata, Xenocephalites ............... 19 Sigaloceras .............................................. 4,8

Pentacrinus ............................................. 10 calloviense ............................. 2, 3, 4,7, 8, 17,27

peramplus, Kepplerites ................................. 27,28 zone ............................................ 5

Perisphinctidae ... i..................................... 1, 2, 4 enodatum ........................................... 33

phillipsi, Xenocephalites ........................... 6, 18, pi. 1 Siliceous material ....................................... 10-11

Phylloceras ........................................3, 7,12,18 Silstone .................................................. 10

Pinna .................................................... 10 Sources of sediment ..................................... 8-9

piperense, Cadoceras ....................... 3, 6, 25, pis. 12-14 stenolobum, Cadoceras ................................... 2

planiventralis, Kepplerites (Seymourltes) ............... 6 Stockade Beaver shale member.........................4, 9,10

planum, Gowericeras .................................... 30 Stratigraphic summary ................................. 4-5

plenus, Kepplerites ...................................... 26 stutchburii, Cosmoeeras (Gulielmiceras) ................ 32

Pleurocephalites ......................................... 5,23 siibitum, Cadoceras ...................................... 28

krylowi ............................................. 5 Gowericeras .... .3, 6, 7,11,17, 25, 27, 28, 29, 30, 31,3P-34,

Procerites ...................................4, 5,11,12,17, 18 pis. 21, 22

funatus ............................................. 33 distinctum, Gowericeras ...................... 6, #S, p1 , 22

tmetolobus .......................................... 33 subpatruum, Cadoceras ............................'. .2,2f«, 23

spp .......................................6, 33, pis. 23, 24 Summary of results .................................... .17-18

Proplanulites ............................................ 5 Sundance formation ...................................4, f, 10

koenigi ..................................... 3, 4, 5, 7, 8, 17 surense, Cadoceras ...................................... f 22

zone ............................................ 5 Sweetgrass Arch ........................................ 8,9

Pseudocadoceras ................................... 2, 7,12,18 Sweetwater County, Wyo. ............................... 9

Q

Quenstedtoceras

R

References 34

3

retrocostatus, Grossouvria ............................... 34

7 tetonense, Cadoceras ....................... 3, 6, 24, pis. 10,11

tmetolobus, Procerites ................................... 33

toricellii, Gowericeras ................................... 31

Trigonia ................................................. 10

tychonis, Kepplerites ........................ 3, 7,11,12, IT, 27

Kepplerites (Seymourites) ...................6, £7, pi, 20

Twin Creek limestone ................................ 5,1C. 11

V

Rierdon formation .................................4, 9,10,11

Gulch ............................................... 11

rierdonense, Arcticoceras ..............2, 6,19,21,22, pis. 2, 3

rockymontanus, Kepplerites ..........................3, 26, 28 Vig0rosum, Cosmoeeras (Gulielmiceras) ............6,32, pi. 9

Kepplerites (Seymourites) .......................... 6

rosenkrantzi, Kepplerites ............................... 27, 28

Kepplerites (Seymourites) ................... 6, #7, pi. 14 W

S

Sand ................................................... 9

San Rafael Swell ........................................ 8

Sawtooth Range ......................................... 9

(Seymourites), Kepplerites .............................. 3

landuskiensis, Kepplerites .................... 6, 27, pi. 19

mclearni, Kepplerites .....................6,25, pis. 15-17

planiventralis, Kepplerites ................... 6, 28, pi. 20

rockymontanus, Kepplerites .............. 6, 26, pis. 18,19

rosenkrantzi, Kepplerites ..................... 6, 27, pi. 14

tychonis, Kepplerites ......................... 6, #7, pi. 20

Shale .................................................... 10

ivarmdomensis, Grossouvria ................... 6, #5, 34, pi. 19

Xenocephalites ......................2, 3, 7, 8, 11,12, 17, IS, 19

bearpawensis .................................. 6,./5, pi. 1

borealis ............................................. 19

nikitini ........................................6,19, pi. 1

parvus angustiumbilicata ........................... 19

phillipsi .......................................6, iS, r1 -1

Z I

zortmanense, Cosmoeeras (Gulielmiceras) ........6, ##, pi. 14

U. S. GOVERNMENT PRINTING OFFICE: 1953——2O9238

PLATES 1-24
PLATE 1

[Figures natural size unless otherwise indicated] 

FIGURES 1, 2, 5, 9. Xenocephalites phillipsi Imlay, n. sp. (p. 18).

I,2, 9. Holotype, U.S.N.M. 108238. From U.S.G.S. Mes. loc. 10730, Rierdon formation. Suture line drawn

near end of chambered shell.

5. Paratype, U.S.N.M. 108239. From U.S.G.S. Mes. loc. 10730, Rierdon formation. 

3,4,6,8. Xenocephalites bearpawensis Imlay, n. sp. (p. 19).

3, 4. Holotype, U.S.N.M. 108240. From U.S.G.S. Mes. loc. 10732, Rierdon formation. 

6. Paratype, U.S.N.M. 108242. From U.S.G.S. Mes. loc. 18758, Rierdon formation. 

8. Paratype, U.S.N.M. 108241. From U.S.G.S. Mes. loc. 10732, Rierdon formation. 

7. Xenocephalites cf. X. nikitini (Burckhardt), (p. 19).

U.S.N.M. 108243. From U.S.G.S. Mes. loc. 18737, Rierdon formation. 

10-13. Lilloettia sp. ind. (p. 18).

10,12. Lateral view of specimen, U.S.N.M. 108236. From U.S.G.S. Mes. loc. 19583, Rierdon foriration.

II,13. Lateral and apertural view of specimen, U.S.N.M. 108237. From U.S.G.S. Mes. loc. 19584, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 1

13

XENOCEPHALITES AND LILLOETTIA
PLATE 2

[Figures natural size unless otherwise indicated] 

FIGURES 1, 2. Arcticoceras rierdonense Imlay, n. sp. (p. 19).

Lateral view and last suture line of holotype, U.S.N.M. 108244. From U.S.G.S. Mes. loc. 19593, Rierdon 

formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 2

ARCTICOCERAS
PLATE 3

[Figures natural size unless otherwise indicated] 

FIGURES 1-6. Arcticoceras crassicostatum Imlay, n. sp. (p. 20).

Iy2. Holotype, U.S.N.M. 108250. From U.S.G.S. Mes. loc. 20496, Sundance formation, Canyon Springs sandstone member.

3, 4. Paratype, U.S.N.M. 108251a. From same locality as holotype.

5, 6. Paratype, U.S.N.M. 108251b. From same locality as holotype. Note that anterior end is directed downward. 

7-12,14. Arcticoceras loveanum Imlay, n. sp. (p. 21).

7, 8. Holotype, U.S.N.M. 108252. From U.S.G.S. Mes. loc. 20496, Sundance formation, Canyon Springs sandstone member.

9,10. Paratype, U.S.N.M. 108253a. From same locality as holotype. 

11,12. Paratype, U.S.N.M. 108253b. From same locality as holotype. 

14. Paratype, U.S.N.M. 108254. From U.S.G.S. Mes. loc. 19556, Sundance formation, Stockade Beaver shale

member. 

13,15,17. Arcticoceras n. sp. ind. (p. 20).

13,15. Specimen, U.S.N.M. 108248. From U.S.G.S. Mes. loc. 19600, Rierdon formation. 

17. Lateral view of specimen, U.S.N.M. 108249. From U.S.G.S. Mes. loc. 18610, Rierdon formation. 

16,18, 21,22. Arcticoceras rierdonense Imlay, n. sp. (p. 19).

16. Suture line of paratype, U.S.N.M. 108245a. From U.S.G.S. Mes. loc. 18610, Rierdon formation. Suture

line drawn at whorl height of 96 mm.

18. Paratype, U.S.N.M. 108246. From U.S.G.S. Mes. loc. 19397, Rierdon formation. 

21, 22. Paratype, U.S.N.M. 108245b. From U.S.G.S. Mes. loc. 18610, Rierdon formation. 

19, 20. Arcticoceras sp. juv. cf. A. rierdonense Imlay, n. sp. (p. 20).

U.S.N.M. 108247a, b. From U.S.G.S. Mes. loc. 18610, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 3

22

ARCTICOCERAS
PLATE 4

[Figures natural size unless otherwise indicated] 

FIGURES 1-13. Arcticoceras codyense Imlay (p. 21).

1. 4, 6. Holotype, U.S.N.M. 104132. From U.S.G.S. Mes. loc. 17106, Sundance formation.

2. 3. Plesiotype, U.S.N.M. 108257a. From U.S.G.S. Mes. loc. 20497, Sundance formation, Canyon Springs

sandstone member. 

5. Suture line of paratype 104133a. From U.S.G.S. Mes. loc. 19589, Rierdon formation. Suture line drawn at

whorl height of 110 mm. 

7,8. Plesiotype, U.S.N.M. 108256. From U.S.G.S. Mes. loc. 20496, Sundance formation, Canyon Springs

sandstone member.

9,12. Paratype, U.S.N.M. 104133b. From U.S.G.S. Mes. loc. 19589, Rierdon formation. 

10,11. Plesiotype, U.S.N.M. 108257b. From U.S.G.S. Mes. loc. 20497, Sundance formation, Canyon Springs

sandstone member. 

13. Sectional diagram of paratype, U.S.N.M. 104133c. From U.S.G.S. Mes. loc. 19589, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 4

ARCTICOCERAS
PLATE 5

[All figures natural size] 

FIGURES 1-5. Cosmoceras (Gulielmiceras) knechteli Imlay, n. sp. (p. 31).

I, 2. Paratype, U.S.N.M. 104151. From U.S.G.S. Mes. loc. 19211, Rierdon formation.

3. Paratype, U.S.N.M. 108298. From U.S.G.S. Mes. loc. 19203, Rierdon formation.

4. 5. Holotype, U.S.N.M. 104152. From U.S.G.S. Mes. loc. 18726, Rierdon formation. 

6-15. Arcticoceras henryi (Meek and Hayden) (p. 22).

6. Plesiotype, U.S.N.M. 108258a; 7, plesiotype, U.S.N.M. 108258b; 10, plesiotype, U.S.N.M. 108258c; 12,

plesiotype, U.S.N.M. 108258d, showing ventral lappet; 14, plesiotype, U.S.N.M. 108258e; 13, 15, plesiotype,

U.S.N.M. 108258f, showing adult body chamber. All from U.S.G.S. Mes. loc. 19561, Sundance formation,

Stockade Beaver shale member. 

8,9. Plesiotype, U.S.N.M. 108259. From U.S.G.S. Mes. loc. 19556, Sundance formation, Stockade Beaver

shale member.

II. Holotype, U.S.N.M. 314. From southwest base of Black Hills, South Dakota. Sundance formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 5

15

COSMOCERAS AND ARCTICOCERAS
PLATE 6

[All figures natural size] 

FIGURES 1-14. Cadoceras shoshonense Imlay (p. 23).

1,2. Plesiotype, U.S.N.M. 108261a; 3, 4, plesiotype, U.S.N.M. 104145a; 5, 6, plesiotype, U.S.N.M. 108261b;

7, 11, plesiotype, U.S.N.M. 104145b. All from U.S.G.S. Mes. loc. 19169, Rierdon formation. 

8. Plesiotype, U.S.N.M. 104142. From U.S.G.S. Mes. loc. 19179. 

9,13,14. Last suture line and ventral and lateral views of holotype, U.S.N.M. 104143. From U.S.G.S. Mes.

loc. 17179, Sundance formation. 

10,12. Ventral views of paratypes, U.S.N.M. 104144a, b. From U.S.G.S. Mes. loc. 19180, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 6

14

CADOCERAS
PLATE 7

[Figure four-fifths natural size] 

Cadoceras muelleri Imlay, n. sp. (p. 23).

Lateral view of holotype showing large smooth body chamber. U.S.N.M. 108262. From U.S.G.S. Mes. loc. 19182, Rierdon 

formation. Other views of holotype on pi. 8, figs. 2, 7, and pi. 9, figs. 8, 10.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 7

CADOCERAS
PLATE 8

[Figures natural size unless otherwise indicated] 

FIGURES 1, 6, 8. Cosmoceras (Gulielmites) of. C. jason (Reinecke) (p. 33).

Lateral and ventral views of specimen, U.S.N.M. 108306. From U.S.G.S. Mes. loc. 19196, Rierdon formation. 

2, 7. Cadoceras muelleri Imlay, n. sp. (p. 23).

Penultimate whorl and partial suture line of holotype, U.S.N.M. 108262. From U.S.G.S. Mes. loc. 19182,

Rierdon formation. Other views of holotype on pi. 7, fig. 1, and pi. 9, figs. 8, 10. 

3-5. Cosmoceras (Gulielmites) cf. C. obductum (Buckman) (p. 33).

3. Lateral view of part of body chamber, U.S.N.M. 108305. From U.S.G.S. Mes. loc. 10729, Rierdon formation.

4. 5. Ventral and lateral views of specimen, U.S.N.M. 108304. From U.S.G.S. Mes. loc. 18735, Rierdon

formation. 

9. Cadoceras muelleri Imlay, n. sp. (p. 23).

Paratype, U.S.N.M. 108265. From U.S.G.S. Mes. loc. 19170, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 8

COSMOCERAS AND CADOCERAS
PLATE 9

[Figures natural size unless otherwise indicated] 

FIGURES 1-3, 6-8,10. Cadoceras muelleri Imlay, n. sp. (p. 23).

1. 3. Paratype, U.S.N.M. 108263. From U.S.G.S. Mes. loc. 19181, Rierdon formation.

2. 6. Paratype, U.S.N.M. 108264. From U.S.G.S. Mes. loc. 19179, Rierdon formation.

7. Suture line at whorl height of about 30 mm. on paratype, U.S.N.M. 108266. From U.S.G.S. Mes.

loc. 19632. 

8,10. Inner whorls of holotype, U.S.N.M. 108262. From U.S.G.S. Mes. loc. 19182, Rierdon formation.

Other views on pi. 7, fig. 1, and pi. 8, figs. 2, 7. 

4,5,9. Cosmoceras (Gulielmiceras) vigorosum Imlay, n. sp. (p. 32).

Lateral and ventral vi'ews of holotype, U,S.N.M. 108303. From U.S.G.S. Mes. loc. 18754, Rierdon

formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 9

10
PLATE 10

[All figures natural size] 

FIGURES 1, 2. Cadoceras tetonense Imlay, n. sp. (p. 24).

Lateral and apertural views of holotype, U.S.N.M. 108267. From U.S.G.S. Mes. loc. 19593, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 10

CADOCERAS
PLATE 11

[Figures natural size unless otherwise indicated] 

FIGURES 1-10. Cadoceras tetonense Imlay, n. sp. (p. 24).

1. 3, 9. Paratype, U.S.N.M. 108271. From U.S.G.S. Mes. loc. 19179, Rierdon formation.

2. Suture line at whorl height of 95 mm. near body chamber on paratype, U.S.N.M. 108268. From U.S.G.S. 

Mes. loc. 19594, Rierdon formation.

4. 7. Lateral and ventral views of paratype, U.S.N.M. 108269. From U.S.G.S. Mes. loc. 19202, Rierdon 

formation.

5. 6. Paratype, U.S.N.M. 108270a. From U.S.G.S. Mes. loc. 19169, Rierdon formation. 

8,10. Paratype, U.S.N.M. 108270b. From U.S.G.S. Mes. loc. 19169, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 11

10

CADOCERAS
PLATE 12

I Nine-tenths natural size] 

Cadoceras piperense Imlay, n. sp. (p. 25).

Paratype U.S.N.M. 108273. From U.S.G.S. Mes. loc. 19213, Rierdon formation. Note suture line on pi. 13, fig. 8.
GEOLOGICAL, SURVEY PROFESSIONAL, PAPER 249 PLATE 12

CADOCERAS
PLATE 13

[Figures natural size unless otherwise indicated] 

FIGURES 1-12. Cadoceras piperense Imlay, n. sp. (p. 25).

I, 2. Paratype, U.S.N.M. 108275a. From U.S.G.S. Mes. loc. 19169, Rierdon formation. 

3,4. Paratype, U.S.N.M. 108277. From U.S.G.S. Mes. loc. 19628, Rierdon formation. 

5,6. Paratype, U.S.N.M. 108276. From U.S.G.S. Mes. loc. 19170, Rierdon formation.

7. 9. Paratype, U.S.N.M. 108278. From U.S.G.S. Mes. loc. 19632, Sundance formation.

8. Suture line at diameter of 187 mm. on paratype, U.S.N.M. 108273. From U.S.G.S. Mes. loc. 19213, Rierdon formation. Same paratype shown on pi. 12. 

10,12. Paratype, U.S.N.M. 108275b. From U.S.G.S. Mes. loc. 19169, Rierdon formation.

II. Paratype, U.S.N.M. 108274. From U.S.G.S. Mes. loc. 19180, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 13

• -'^i^ •"'" n,'.~ %^~f

• vtv--- t_.,^ •'^sc^'^^p^'-

•': '-..-_ •-' "-"• •K*r .^..7':ls54;-'.".52i-''i«r5<

.- i*' -••?" •-'"" "5":*'—-fe ^--^'.^ >£r^ ;-^-v"-ie -' i*"^3

.v^?t ;-^ • x - ^.^y^i>,^J^:'

s*r- 10 12

CADOCERAS
PLATE 14

I All figures natural size] 

FIGURES 1-6, 8. Cosmoceras (Gulielmiceras) zortmanense Imlay, n. sp. (p. 32).

1. Paratype, U.S.N.M. 108302a. From U.S.G.S. Mes. loc. 18750, Rierdon formation.

2. Paratype, U.S.N.M. 108301. From U.S.G.S. Mes. loc. 18742, Rierdon formation. Note lateral lappet.

3. 4. Paratype, U.S.N.M. 108300. From U.S.G.S. Mes. loc. 18737, Rierdon formation.

5. Rubber mold of holotype, U.S.N.M. 108299. From U.S.G.S. Mes. loc. 18737, Rierdon formation.

6. 8. Ventral 'and lateral views of paratype, U.S.N.M. 108302b. From U.S.G.S. Mes. loc. 18750, Rierdon

formation. 

7. Cadoceras piperense Imlay, n. sp. (p. 25).

Holotype, U.S.N.M. 108272. From U.S.G.S. Mes. loc. 19599, Rierdon formation. Note body chamber is

nearly complete. 

9. Kepplerites (Seymourites) cf. K. rosenkrantzi Spath (p. 27).

U.S.N.M. 108279. From U.S.G.S. Mes. loc. 7124, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 14

COSMOCERAS, CADOCERAS, AND KEPPLERITES
PLATE 15

[Two-thirds natural size] 

Kepplerites (Seymourites) mclearni Imlay (p. 25).

Holotype, U.S.N.M. 104125. From U.S.G.S. Mes. loc. 19211, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 15

KEPPLERITES
PLATE 16

[Three-fifths natural size] 

Kepplerites (Seymourites) mclearni Imlay (p. 25).

U.S.N.M. 104130. From U.S.G.S. Mes. loc. 19584, Rierdon formation. Note that body chamber is represented by 1% whorls.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 16

KEPPLERITES
PLATE 17

[All figures natural size] 

FIGURES 1-8. Kepplerites mclearni Imlay (p. 25).

1. Paratype, U.S.N.M. 104127. From U.S.G.S. Mes. loc. 19584, Rierdon formation.

2. 8. Paratype, U.S.N.M. 104122. From U.S.G.S. Mes. loc. 10732, Rierdon formation.

3. 6. Paratype, U.S.N.M. 104128. From U.S.G.S. Mes. loc. 18750, Rierdon formation.

4. 5. Paratype, U.S.N.M. 104126. From U.S.G.S. Mes. loc. 19583, Rierdon formation. 

7. Paratype, U.S.N.M. 104123. From U.S.G.S. Mes. loc. 18737, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 17

KEPPLER1TES
PLATE 18

[Four-fifths natural size] 

Kepplerites (Seymourites) rockymontanus Imlay, n. sp. (p. 26).

Holotype, U.S.N.M. 104131. From U.S.G.S. Mes. loc. 19211, Rierdon formation. Note that incomplete body chamber is 

represented by a complete whorl.
GEOLOGICAL SUKVEY PROFESSIONAL PAPER 249 PLATE 18

KEPPLERITES
PLATE 19

[Figures natural size unless otherwise indicated] 

FIGURES 1, 2. Kepplerites (Seymourites) rockymontanus Imlay, n. sp., (p. 26).

Lateral view and suture line of paratype, U.S.N.M. 104146. From U.S.G.S. Mes. loc. 10730, Rierdon formation. Suture line drawn from a fragment larger than the one figured. 

3,6-8. Kepplerites (Seymourites) landuskiensis Imlay, n. sp. (p. 27).

3, 8. Lateral views of the penultimate and body whorls of paratype, U.S.N.M. 108281. From U.S.G.S. Mes.

loc. 18726, .Rierdon formation. Note aperture is essentially complete. 

6, 8. Holotype, U.S.N.M. 108280. From U.S.G.S. Mes. loc. 18726, Rierdon formation. 

4, 5. Grossouvria warmdomensis Imlay, n. sp. (p. 33).

Lateral and ventral views of holotype, U.S.N.M. 108309. From U.S.G.S. Mes. loc. 19585, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 19

KEPPLERITES AND GROSSOUVRIA
PLATE 20

[All figures natural size] 

FIGURES 1-3. Kepplerites tychonis Ravn (p. 27).

1, 2. Plesiotype, U.S.N.M. 104173. From U.S.G.S. Mes. loc. 19629, Rierdon formation.

3. Plesiotype, U.S.N.M. 104172. From U.S.G.S. Mes. loc. 20390, Rierdon formation. Aperture is nearly

complete. 

4-9. Kepplerites (Seymourites) planiventralis Imlay, n. sp. (p. 28).

4. 5, 8. Ventral and lateral views of paratype, U.S.N.M. 108284. From U.S.G.S. Mes. loc. 18754, Rierdon 

formation. Note ventral flattening. Figure 4 is oriented with anterior end down.

6. Paratype, U.S.N.M. 108283. From U.S.G.S. Mes. loc. 19211, Rierdon formation.

7. 9. Holotype, U.S.N.M. 108282. From U.S.G.S. Mes. loc. 19211, Rierdon formation. Shows one-half whorl 

of body chamber.
PROFESSIONAL PAPER 249 PLATE 20

KEPPLERITES
PLATE 21

[All figures natural size] 

FIGURES 1-18. Gowericeras subitum Imlay (p. 28).

I. Lateral view of plesiotype, U.S.N.M. 108287a. From U.S.G.S. Mes. loc. 19180, Rierdon formation.

2,3. Paratype, U.S.N.M. 104156c; 5, paratype, U.S.N.M. 104156b; 8, 9, paratype, U.S.N.M. 104156a. All

from U.S.G.S. Mes. loc. 19182, Rierdon formation. 

4,10. Paratypes, U.S.N.M. 104153a, b. From U.S.G.S. Mes. loc. 19169, Rierdon formation.

6. Plesiotype, U.S.N.M. 108286. From U.S.G.S. Mes. loc. 19179, Rierdon formation.

7. Plesiotype, U.S.N.M. 108285a, From U.S.G.S. Mes. loc. 19169, Rierdon formation.

II,12. Plesiotype, U.S.N.M. 108285b. From U.S.G.S. Mes. loc. 19169, Rierdon formation. 

13,14. Holotype, U.S.N.M. 104155. From U.S.G.S. Mes. loc. 19182, Rierdon formation. 

15,16. Plesiotype, U.S.N.M. 108287b. From U.S.G.S. Mes. loc. 19180, Rierdon formation. 

17,18. Paratype, U.S.N.M. 104154. From U.S.G.S. Mes. loc. 19180, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPEK 24« PLATE 21

18

GOWERICERAS
PLATE 22

[All figures natural size! 

FIGURES 1-4, 7, 8. Gowericeras costihians Imlay, n. sp. (p. 29).

1, 2. Paratype, U.S.N.M. 108290. From U.S.G.S. Mes. loc. 19182, Rierdon formation. 

3,4,7. Holotype, U.S.N.M. 108289. From U.S.G.S. Mes. loc. 19182, Rierdon formation. Suture lines 

drawn at whorl height of 19 mm immediately preceding body chamber. 

8. Paratype, U.S.N.M. 108314. From U.S.G.S. Mes. loc. 19182, Rierdon formation. 

5, 6. Gowericeras subitum var. distinctum Imlay, n. var. (p. 29).

Holotype, U.S.N.M. 108288. From U.S.G.S. Mes. loc. 19179, Rierdon formation. Shows complete body

chamber. 

9. Gowericeras subitum Imlay (p. 28).

Plesiotype, U.S.N.M. 108285c. From U.S.G.S. Mes. loc. 19169, Rierdon formation. Shows sparser, sharper

ribbing than most immature forms of this species. 

10-13. Gowericeras costidensum Imlay, n. sp. (p. 31).

10,11. Holotype, U.S.N.M. 108297. From U.S.G.S. Mes. loc. 19212, Rierdon formation. 

12,13. Paratype, U.S.N.M. 108296. From U.S.G.S. Mes. loc. 19170, Rierdon formation.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 22

13

GOWERTCERAS
PLATE 23

[Figures natural size unless otherwise indicated] 

FIGURES 1-12,18. Gowericeras costicrassum Imlay, n. sp. (p. 30).

1-3. Holotype, U.S.N.M. 108294. From U.S.G.S. Mes. loc. 19170, Rierdon formation. Shows complete

body chamber.

4. Paratype, U.S.N.M. 108292. From U.S.G.S. Mes. loc. 19180, Rierdon formation. 

5-7. Paratype, U.S.N.M. 108295a. From U.S.G.S. Mes. loc. 19170, Rierdon formation. 

8, 9. Paratype, U.S.N.M. 108293a. From U.S.G.S. Mes. loc. 19182, Rierdon formation. Suture line drawn

at whorl height of 9 mm.

10,11. Paratype, U.S.N.M. 108293b. From U.S.G.S. Mes. loc. 19182, Rierdon formation. 

12,18. Paratype, U.S.N.M. 108295b. From U.S.G.S. Mos. loc. 19170, Rierdon formation. 

13,17. Procerites sp. (p. 33).

Ventral and lateral views of specimen, U.S.N.M. 108307. From U.S.G.S. Mes. loc. 18718, Rierdon formation. 

14-16. Gowericeras costimedium Imlay, n. sp. (p. 30).

Holotype, U.S.N.M. 108291. From U.S.G.S. Mes. loc. 19180, Rierdon formation. Suture line drawn at 

whorl height of 17 mm. near beginning of body chamber.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 23

GOWERICERAS AND PROCERITES
PLATE 24

[Figures natural size unless otherwise indicated] 

FIGURES 1,3,8. Grossouvria sp. (p. 34).

Lateral views of specimens, U.S.N.M. 108310a-c. From U.S.G.S. Mes. loc. 21467, Rierdon formation. 

2,4-6. Grossouvria sp. (p. 34).

2, 6. Lateral and ventral views of specimen, U.S.N.M. 108312. From U.S.G.S. Mes. loc. 19182, Rierdon

formation. 

4,5. Lateral and ventral views of specimen, U.S.N.M. 108313. From U.S.G.S. Mes. loc. 19180, Rierdon

formation. 

7. Grossouvria sp. (p. 34).

Specimen, U.S.N.M. 108311. From U.S.G.S. Mes. loc. 20365, Rierdon formation. 

9-11. Procerites sp. (p. 33).

Specimen, U.S.N.M. 108308. From U.S.G.S. Mes. loc. 19603, Rierdon formation. Suture line drawn at 

whorl height of 64 mm.
OEOLOGICAL SURVEY PROFESSIONAL PAPER 249 PLATE 24

GROSSOUVRIA AND PROCERITES