Miocene Marine Diatoms 

From the Choptank Formation, 

Calvert County, Maryland 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 

Miocene Marine Diatoms 

From the Choptank Formation, 

Calvert County, Maryland 

By GEORGE W. ANDREWS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 

A description of a 1niddle hi iocene 

marine diatonl assentblage for the 

Atlantic Coastal Plain region 

and its stratigraphic and 

paleoecologic significance 

UNITED STATES GOVERN"IENT PRI~ TI~G OFFICE, '\\rASHINGTON 1976 
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOIVIAS S. KLEPPE, Secretary 

GEOLOGICAL SURVEY 

V. E. l\icKelvey, Director 

Library of Congress Cataloging in Publication Data 

Andrews, George William, 1929-

Miocene marine diatoms from the Choptank Formation, Calvert County, Maryland. 

(Geological Survey professional paper ; 910) 

Bibliography: p. 

Includes index. 

Supt. of Docs. no.: I 19.16:910 

1. Diatoms, Fossil. 2. Paleontology-Miocene. 3. Paleontology-Maryland-Calvert Co. I Title II. Series: United 

States. Geological Survey. Professional paper ; 910. 

QE955.A62 561'.93 74-34195 

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

Washington, D.C. 20402 

Stock Number 024-001-02735-6 
PLATE 

FIGURE 

TABLE 

1. 

2. 

3. 

4. 

5. 

6. 

CONTENTS 

Page 

Abstract ----------------------------------------------------------------- 1 

Introduction -------------------------------------------------------------- 1 

Acknowledgments --------------------------------------------------------- 1 

Miocene stratigraphy ____________________________________ - ----------------- 2 

Calvert Cliffs locality _ ___ _ _ __ _ ___ _ __ ____ _ __ __ _ __ _ __ __ _ __ _____ _ ___ _ ____ ____ 2 

Previous work on diatoms of the Chesapeake Group -------------------------- 4 

Diatom assemblage ------------------------------------------------------- 6 

Systematic descriptions ________________________________ -------------------- 7 

Selected references -------------------------------------------------------- 23 

Index ------------------------------------------------------------------- 25 

ILLUSTRATIONS 

[Plates follow index I 

Melosira, Paralia, Pyxidicula, and Stephanopy;r;is. 

Stephanopyxis and Coscinodiscus. 

Coscinodiscns, Craspedodiscus, and Actinocyclus. 

H yalodiscus and Actinoptyclws. 

Rattrayella, Liradiscus, Biddulphia, Goniothecium, Encampia, Terpsinoe, Trice'ratium, and Periptera. 

Xanthiopyxis, Dossctia, Stephanogonia, Psemlopyxilla, and Rhaphoneis. 

7. Rhaphoncis, Thalassioncma, Tlwlassiothrix, Grammatophora, Cocconcis, Na'vicula,, Diploneis, Pleurosigma, and 

Amphora. 

Page 

1. Index map showing location of the Calvert Cliffs area in Maryland --------------------------------- 3 

2. Map showing location of the Choptank fossil diatom locality, Calvert County, Md -------------------- 4 

TABLE 

Page 

1. Comparison of diatom taxa from the Chesapeake Group. ___________________________________________ _ 7 

III 

MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION, 

CALVERT COUNTY, MARYLAND 

By GEORGE W. ANDRE\VS 

ABSTRACT 

The Choptank Formation of Miocene age contains diatombearing strata in outcrops in the Calvert Cliffs on the west 

side of Chesapeake Bay. A good diatom assemblage is present in the upper part (stratigraphic zone 19 of Shattuck 

[1904] ) in the section exposed immediately north of the Calvert Cliffs nuclear power plant, Calvert County, Md. 

The Choptank Formation has long been correlated with the 

middle Miocene of the European section principally on the 

basis of its molluscan fauna. Recent studies of planktonic 

Foraminifera suggest that it correlates with either the upper 

Langhian, the Serravalian, or the Tortonian Stages (middle 

to upper Miocene) , but a more precise determination has not 

yet been made. The macrofauna and microfauna of the formation are suggestive of deposition in a shallow, inner-shelf 

marine environment. The diatom assemblage in the upper part 

of the Choptank Formation reinforces these conclusions. 

There are fewer extinct taxa and more still-living taxa in the 

Choptank assemblage than in the older assemblages present 

in the underlying Calvert Formation. The Choptank assemblage consists of exclusively marine diatom species and is 

dominated by the more robust centric planktonic diatoms. The 

presence of these diatoms is indicative of a shallow, agitated 

depositional environment and the fact that only a low percentage of benthonic species is present suggests muddy bottom conditions and a shifting substratum. 

Sixty taxa of marine diatoms have been identified in this 

Choptank 'assemblage; seven are previously unreported, 18 are 

extinct forms known only from Miocene rocks, six are extinct forms ranging outside Miocene rocks, and 29 are longranging forms still living in modern oceans. This study of the 

Choptank diatoms adds to our knowledge of the stratigraphic 

ranges of diatom species and thus increases their usefulness 

in stratigraphic correlation and geologic age determination 

of Tertiary strata. 

INTRODUCTION 

This report is the first part of a study of fossil 

marine diatoms in the Tertiary strata of the Atlantic Coastal Plain. The Calvert, Choptank, and St. 

Marys Formations, all part of the Chesapeake 

Group, crop out as a sequence of clastic deposits 

along the Atlantic Coastal Plain of Maryland and 

Virginia. The Calvert Formation is highly to moderately diatomaceous, the Choptank Formation is 

sparingly diatomaceous, and diatoms have not yet 

been found in the St. Marys Formation of Maryland. The upper shell bed of the Choptank Formation, exposed in the Calvert Cliffs southeast of Long 

Beach, Calvert County, Mel., contains a diatom assemblage of 60 taxa; this assemblage is the specific 

subject of this report. The assemblage is distinctly more modern in aspect than those reported previously from the Calvert Formation, and it is considered to be a representative middle Miocene diatom assemblage developed in a shallow marine shelf 

environment. It occurs in a formation of fairly well 

determined stratigraphic position (specifically, the 

base of lithologie unit 18 of Gernant [ 1970, p. 7 4 ] 

from stratigraphic zone 19 of Shattuck [ 1904 ] of 

the Choptank Formation) and hence should be adequate to serve the purpose of this study-to provide 

data on the occurrence of fossil marine diatoms 

from a stratum of known geologic age and to n1ake 

these data available for regional geologie correlation and elating. 

i\CKNOWLEDG i\IENTS 

Access to the collection locality was made possible 

through the kind cooperation of Robert E. Gernant, University of Wisconsin-Milwaukee and Lincoln Dryden, Bryn Mawr College, who were in 

charge of the Calvert Cliffs Project sponsored by 

the Maryland Academy of Sciences. John L. Stone, 

Washington, D.C., prepared several slides for the 

writer with Naphrax, 1 a high-index mounting mediunl; in addition, he searched a number of slides 

for additional specimens and provided helpful discussion of the diatom identification and taxonomy. 

Kenneth E. Lohman, Division of Paleobotany, The 

Smithsonian Institution, Blake Blackwelder, Joseph 

E. Hazel, and Sergius H. Mamay, U.S. Geological 

Survey, have reviewed the manuscript and their 

many helpful comments are appreciated. 

1 Anv use of tJ·a(le names in this publication i,; for descriptive purposes 

only a'Od does not constitute endorsement by the U.S. Geological Survey. 

1 
2 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

lVIIOCENE STRATIGRAPHY 

The Miocene strata of the Middle Atlantic States 

have been well known since early colonial times because of their excellent exposure along Chesapeake 

Bay in Maryland and along the Potomac River estuary in Virginia. Shattuck (1904, p. 41-64) gives 

an exhaustive bibliography of publications on the 

Miocene strata of Maryland for the years 1669-

1903. Modern stratigraphic terminology began with 

Darton (1891, p. 433), who named the entire Miocene stratigraphic section of the region the Chesapeake Formation. This name was modified by Dall 

and Harris ( 1892, p. 123) to Chesapeake Group and 

was extended to include all Miocene strata of the 

Atlantic Coastal Plain. The Chesapeake Group in 

Maryland was first divided into the Calvert, Choptank, and St. Marys Formations by Shattuck (1902). 

The Chesapeake Group in Maryland was described 

in detail by ShaUuck ( 1904, p. 69-92) , and the systematic paleontology of various fossil groups of the 

Miocene deposits of Maryland was also presented 

in that volume, including a chapter by C. S. Boyer 

(1904) on the diatoms. Shattuck (1904) divided the 

Calvert Formation into a lower Fairhaven Diatomaceous Earth Member and an upper Plum Point Marl 

Member. In addition, he divided the entire Miocene 

section into 24 numbered stratigraphic zones. Of 

these, zones 1-3 were referred to the Fairhaven of 

the Calvert Formation, zones 4-15 to the Plum 

Point of the Calvert Formation,. zones 16-20 to the 

Choptank Formation, and zones 21-24 to the St. 

Marys Formation. Gernant (1970) assigned names 

of member rank to the numbered zones of Shattuck 

(1904) for the Choptank Formation. 

The Chespeake Group of Maryland, in general, 

consists of a series of sands, silts, and clays containing abundant macrofossils and microfossils in certain beds. Practically all the strata consist of mixtures of sand, silt, and clay in varying amounts, so 

that the beds differ in degree, rather than in kind. 

The deposits are generally blue gray in fresh exposures, but rapidly weather to slate gray. Organic. 

matter is common, and dissociation of the sediments 

for diatom study has indicated traces of hydrocarbons. Traces of iron are present throughout the section and probably account for the change of color 

upon weathering. Calcium carbonate appears to occur only as fossils or fossil fragments, and there 

seems to be little evidence of primary deposition. 

The few hard calcareous beds in the section are 

associated with the shell beds; the abundance of 

shell molds in these cemented strata indicates leaching of the shells and secondary carbonate deposition. 

The Fairhaven Diatomaceous Earth Member of 

the Calvert Formation rests unconformably on underlying Eocene strata. It may be readily distinguished from the Eocene deposits, which contain 

abundant glauconite and quartz granules but lack 

diatoms. The Fairhaven consists mainly of an impure silty diatomite. The overlying Plum Point Marl 

Member of the Calvert Formation consists of a 

series of impure fine sands, silts, and clays and beds 

composed primarily of mollusk shells in a sand-siltclay matrix. This part of the Calvert Formation is 

moderately diatomaceous. 

The Choptank Formation is said by Shattuck 

(1904, p. 80) to rest unconformably upon the Calvert Formation. Although this may be apparent on 

a regional basis, the Choptank lithology is so similar 

to that of the upper part of the Calvert Formation 

that an exact determination of the contact is difficult in most outcrops. Two parts of the Choptank 

Formation, stratigraphic zones 17 and 19, are distinguished primarily by their high content of mollusk shells. The Choptank Formation is part barren 

of diatoms and sparingly diatomaceous in some beds 

of all its members. 

The St. Marys Formation rests conformably on 

the Choptank Formation according to Shattuck 

(1904, p. 84-85). Gernant (1970, p. 8-10), however, 

cites evidence for a surface of either erosion or nondeposition on the top of the Choptank Formation 

and, therefore, postulates an unconformable contact 

with the St. Marys Formation. The St. Marys Formation consists of four recognized zones of clastic 

sediments in the Chesapeake Bay region. Up to the 

present I have found no diatoms in the St. Marys 

Formation in samples from zones 22 and 23 at the 

Calvert Cliffs locality or from the Chancellors Point 

or Windmill Point localities, St. Marys County, Mel. 

CALVERT CLIFFS LOCALITY 

The exposure of the Choptank Formation from 

which the studied diatom assemblage was obtained 

is on the western shore of Chesapeake Bay in the 

Cove Point 71·:!-minute quadrangle, Calvert County, 

Md. The location of the Calvert Cliffs area in Maryland is shown on the index map (fig. 1). During an 

early stage of construction of the Calvert Cliffs nuclear power plant of the Baltimore Gas and Electric 

Co., an area of the cliffs about 200 yards (183 m) 

northwest of the plant site was cleared of surficial 

debris for the stratigraphic and paleontologic studies 

conducted by the Maryland Academy of Sciences. 
CALVERT CLIFFS LOCALITY 3 

Phi adelphi~ 

PENNSYLVANIA JjjJ 

,. 

------------r NEW 

MARYLAND 

VIRGINIA 

Richmond 0 

Base from 1:3,168,000 Un ited States 

Base map, redrawn by author 

JERSEY 

1 

1 

L..:....--

lvert Cliffs 

m locality 

FIGURE I.- Location of the Calvert Cliffs area in Maryland. 

The site is at the end of a line of nearly continuous 

cliffs extending in a southeasterly direction from 

Flag Ponds, and it is approximately 2.0 miles (3.2 

km) southeast of Long Beach, Md., at !at 38° 26' 

16" N., and long 76° 26' 40" W. The collection locality is shown in detail in figure 2. 

Stmti,r;mpliy.-The rocks exposed at the Calvert 

Cliff's power-plant locality range from the upper 

part of zone 17 of Shattuck [1904], into the St. 

Marys Formation (zone 23 of Shattuck [1904]). The 

Miocene section is overlain unconformably by Pleistocene sand. A detailed measured section of the 

Choptank Formation at this locality has been published by Gernant (1970, p. 73-76) as his locality 

66- 25 (Flag Pond II). Gernant measured the total 

thickness for the Choptank Formation here as 39 

feet 1 inch, but recognized that zone 16 and part of 

zone 17 are below sea level and, hence, not exposed. 

The measured thicknesses of the members of the 

Choptank Formation are: zone 17, 1 foot exposed; 

zone 18, 15 feet 4 inches; zone 19, 13 feet 2 inches; 

zone 20, 9 feet 7 inches. 

The Choptank Formation exposed at this section 

was further divided by Gernant (1970) into 21 

lithologic units. Of these, units 11 through 19 make 

up stratigraphic zone 19. The diatom assemblage 

studied for this report was collected at the base of 

Gernant's unit 18, or approximately 16 inches below 

the top of stratigraphic zone 19. Gernant ( 1970, p. 

7 4) describes the lithology of his unit 18 as follows: 

"Reddish-brown muddy, fine sand. Oxidation causing color mottling with various browns, reds, and 

oranges. Contacts slightly undulating and distinct." 

The sediments, therefore, appear to be somewhat 

oxidized, but the abundance of shells suggests only 

moderate carbonate leaching. 

Diatoms occur sparingly throughout stratigraphic 

zone 16 in the nuclear-power-plant excavation adjacent to the natural exposure in the Calvert Cliffs. 

They were found in internal fillings of pelecypods 

about 1 foot below the top of stratigraphic zone 17. 

No diatoms were found in a sample taken 3 feet 

below the top of zone 18. No diatoms were found 

in samples taken 6 and 10 feet below the top of zone 

19; however, the sample taken about 16 inches below 

the top of zone 19 contained a diatom assemblage 

suitable for study, which is herein reported. Diatoms of sparing occurrence were found in stratigraphic zone 20 in samples taken about 2 and 8 

feet below the top of the zone, but no diatoms 

were found in a sample taken near the top of that 

zone. No diatoms were found in a sample taken 5 

feet above the base of zone 21 nor in samples taken 

1, 2, and 3 feet above the base of zone 23, all in the 

St. Marys Formation. 

Geologic age 1'elations!tips.-The Chesapeake 

Group of Maryland has been considered Miocene in 

age primarily on the basis of molluscan and vertebrate faunas. The consensus, as expressed by Cooke, 

Gardner, and Woodring (1943), has been that the 

Calvert Formation is of early middle Miocene age; 

the Choptank Formation, middle middle Miocene; 

and the St. Marys Formation, lat,e middle Miocene. 

Gibson (1967, and in Gernant and others, 1971) 

indicates that stratigraphic zone 10 of the Plum 

Point Marl Member of the Calvert Formation falls 

in either planktonic foraminiferal zone N. 8 or N. 9 

of Blow (1969), which Berggren (1972), and Berggren and Van Couvering (1973) place near the 

lower-middle Miocene boundary, or about 16 million 

years B. P. J. E. Hazel (oral commun., 1974) suggests that the St. Marys Formation (as the formation name is used in Virginia) probably falls in 

planktonic foraminiferal zone N. 17, which in the 

time-scale of Berggren (1972) and Berggren and 

Van Couvering (1973) is Tortonian or Messinian 
4 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

Base from U.S. Geological Survey 

1:24,000 Cove Po1nt, 1943 0 

0 

76.27 '30" 

1000 2000 3000 4000 FEET 

!000 METRES 

FJGUHE 2.-Location of the Choptank fossil diatom locality, Calvert County, Md. 

in age, that is, about 5 to 8 million years B. P. These 

correlations indicate that the Choptank Formation 

falls within the interval of zones N. 10 to N. 16, but 

a more precise correlation has not been made. The 

stratigraphic position of the formation suggests that 

it probably falls in the Serravalian Stage of middle 

Miocene age, but absolute certainty on this point is 

impossible at this time. 

PREVIOUS WORK ON DIATO:\'TS OF 

THE CHESAPEAKE GROUP 

The discovery of diatoms in the rocks of the 

Chesapeake Group is credited to W. B. Rogers 

(1841), State Geologist of Virginia, who reported 

the occurrence of a "tertiary infusorial stratum" at 

Richmond. Rogers sent samples to J. W. Bailey, 

Professor of Chemistry, Mineralogy, and Geology 

at the U.S. Military Academy; Bailey was apparently 

the first American scientist to develop an interest in 

diatoms. Bailey (1842a) mentioned diatom localities 

at Richmond, Va., and on the Rappahannock River. 

Bailey (1842b, p. 329-331) further reported on 

diatoms, silicofl agellates, radiolaria and sponge spicules from the Chesapeake Group of Virginia. Bailey 

( 1842b, p. 330) also stated that he sent specimens 

of the Richmond deposit to C. G. Ehrenberg in Bcr-
PREVIOUS WORK ON DIATOMS OF THE CHESAPEAKE GROUP 5 

lin. A diatom-assemblage list and discussion of the 

correlation of the deposit was published by Ehrenberg (1843, p. 327-330) in his paper on the microscopical life of South and North America. Bailey 

(1843) presented a paper on microfossils from 

Petersburg, Va., before the Association of American 

Geologists and Naturalists. A fuller account of this 

assemblage and another at Piscataway, Mel., was 

published by Bailey in 1844. 

Bailey evidently sent specimens from Petersburg 

and Piscataway to Ehrenberg, who (1844, p. 68-72) 

published a chart comparing the Richmond, Petersburg, and Piscataway assemblages and commented 

on their geologic correlation. Bailey (1845a) reviewed and summarized this work of Ehrenberg. 

The next event in this active sequence of discovery 

and discussion is a paper by Bailey ( 1845b) comparing fossil diatom assemblages from Virginia, 

Maryland, and "Bermuda," with modern diatoms in 

United States coastal waters. 

The locality reported on by Bailey ( 1845b) as 

"Bermuda" deserves further scrutiny at this time. 

Bailey (p. 323) states of this locality: 

Some months ago I received from M. Tuomey, Esq. of Petersburg, Va., a fine specimen of infusorial earth, labelled 'Tripoli 

from Bermuda'. * * ':' The only information with regard to 

the history of the specimen which I have yet been able to 

obtain is, that Mr. Tuomey received it with its present label 

from some mineralogical correspondent, and that he has no 

doubt that it came from Bermuda. 

Thus the "Bermuda Islands" diatom assemblag-e, 

based on an inadequately labeled specimen of un--

known provenance made by an anonymous collector, 

found its way into the literature. Bailey (1845b, 

p. 329) admits the peculiarity of the occurrence: 

It is also remarkable that a deposit so purely siliceous could 

be formed among the coralline isles of Bermuda. No mention 

of any such "Tripoli" is contained in any of the accounts of 

the geology of those islands which I have yet seen. 

Subsequent investigation of the Bermuda Islan<b 

has shown deposits of coralline rock, but no trace 

whatever of Miocene diatomite. An alternative explanation must be sought. 

There is, in Chesterfield County, about 16 miles 

southeast of Richmond, Va., and somewhat closer 

to Petersburg, an early colonial settlement on the 

bank of the James River called Bern1uda Hundred, 

which has now dwindled to a few houses and a 

historical marker. I have not found an outcrop of 

the Calvert Formation here, but it is highly likely 1 that one does (or did) exist somewhere along the 

banks or nearby bluffs of the meandering James 

River. The "Bermuda" diatom assemblage is essentially identical with the other Calvert diatom assemblages reported by Bailey ( 1845b). In view of 

the ambiguous origin of the sample and the close 

degree of correlation, it seems highly probable that 

the so-called Bermuda assemblage came from Bermuda Hundred, Va., rather than the Bermuda 

Islands. 

Bailey (1845b, p. 323) states that he sent a sample of the "Bermuda" material to Ehrenberg, who 

(1845, p. 257-261) published a species list of microfossils. Unfortunately he published the source unequivocally as "Bermuda Islands," so that the locality error has been perpetuated in the European 

literature. Bailey ( 1849) published one further note 

on the Tertiary Infusoria localities in Maryland. 

With his death in 1857, scientific diatom work in 

the United States came to a near standstill for 

almost 40 years. 

In his "Mikrogeologie" Ehrenberg (1854, pis. 18, 

33) devoted an entire plate to the assemblage from 

Richmond and part of a plate to other Miocene assemblages from Virginia. Much later, Grunow in 

Van Heurck (1880-1885) described taxa from the 

Chesapeake Group of Maryland and Virginia. Raphidodiscus marylandicus, a Miocene index diatom of 

worldwide occurrence, was described by Christian 

(1887) from subsurface Calvert Formation of eastern Maryland. A systematic study of the diatoms of 

the Calvert Formation of Maryland was made by 

Boyer (1904). Although he published two plates of 

the more common and interesting cliatmns, many 

species are not figured and only a few are described. 

The most recent published report on the diatoms of 

the Calvert Formation is that by Lohn1an (1948) 

on assemblages from the Hamn1oncl well in eastern 

Maryland. This is the most complete systematic 

study made to date, and it proved to be a helpful 

guide in the preparation of this report. 

As far as I have been able to determine, all the 

previous diatom studies have probably been made 

on diatoms from the Calvert Formation with the possible exception of assemblages from Petersburg, Va. 

The Choptank Formation does crop out at the Stratford Cliffs, and assemblages from this place were 

reported by Bailey (1845b) and Ehrenberg (1854, 

pl. 33). However, the Calvert Formation is more 

easily accessible at this locality and was probably 

the deposit examined. Diatoms occur in far greater 

abundance in the Calvert Formation than in the 

Choptank Formation, and the Calvert is considerably n1ore widespread and thicker. It is, therefore, 

understandable that the Choptank diatoms have 

been neglected. Diatoms are not common in the St. 

Marys Formation but may be discovered upon 

further investigation. 
6 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

DIATOM ASSEMBLAGE 

The samples from U.S. Geological Survey diatom 

locality 6098 were prepared for diatom study following standard procedures of the U.S. Geological 

Survey as outlined by Lohman (1972). An initial 

treatment with hydrochloric acid removed particulate calcareous material and finely divided iron minerals. Organic matter was removed by heating in 

a solution of potassium permanganate and sulfuric 

acid. Finally, the diatom-sized particles were concentrated by differential settling and deeanting. 

Strewn mounts were made in Caedax and Aroclor, 

and additional mounts in the high-index medium 

N aphrax were supplied by Mr. John Stone. Specimens were located by systematic traversing with 

a mechanical stage, and their locations were recorded as coordinates of the mechanical stage. Eight 

strewn mounts, 18 mm in diameter, were examined. 

The relative abundance of each of the 60 diatom 

taxa is shown in the list below, in which the estimated relative frequency under an 18-mm cover 

glass viewed at x 250 magnification is defined as 

follows: abundant (A), at least one specimen in all 

fields of view; common (C) , one specimen in many 

(but not all) fields of view; frequent (F), several 

specimens observed on slide, but seen only in a few 

fields of view; rare (R) one or two specimens on a 

slide. An asterisk ( *), precedes the 19 taxa which 

are extinct forms known only fron1 Miocene rocks; 

most of these forms have been previously reported 

from the Calvert Formation and some have been 

reported from other Miocene localities. A dagger 

( t) precedes the seven taxa which are previously 

unreported; in addition to these, numerous forms of 

Xanthiopyxis show great variation but occur in 

quantities too small for taxonomic treatment. A 

double dagger (:j:) precedes the six taxa which are 

extinct forms having a known range not limited to 

the Miocene; all forms range from Miocene to Pliocene except Liradiscus ovalis and Triceratiurn condccornm, which range from Eocene to Miocene. Thus, 

of the 60 taxa represented in the Choptank assemblage and listed below, 31 (51.7 percent) are 

extinct forms and the remaining 29 ( 48.3 percent) 

are long-ranging forms still living in modern oceans. 

Melosira westii W. Smith ___________________________ C 

':'Pamlia complc.ra (Lohman) Andrews, n. comb ________ R 

sulcata (Ehrenberg) Cleve ---------------------- A 

sulcata var. coronata (Ehrenberg) Grunow _______ A 

*Py.ridicula cruciata Ehrenberg ---------------------- F 

*Stcplwnopy;ris corona (Ehrenberg) Grunow ___________ R 

* linccda (Ehrenberg) Forti ---------------------- F 

tun·is (Greville and Arnott) Ralfs _______________ R 

Coscinodiscus apiculatus Ehrenberg ------------------ R 

curvatulus Grunow ----------------------------- R 

inclusus Rattray ------------------------------- R 

·marginatus Ehrenberg -------------------------- R 

ocul11s-iridis Ehrenberg ------------------------- R 

paforatus Ehrenberg -------------------------- R 

pnforatus var. cellulosa Grunow ----------------- R 

perfomtus var. pavillard1' (Forti) Hustedt ------- R 

radiatus Ehrenberg ---------------------------- R 

rothii (Ehrenberg) Grunow -------------------- R 

:j: ·vetustissimus Pantocsek ------------------------ R 

*Craspedodisclls coscinodiscus Ehrenberg -------------- R 

*Actinocyclns ellipticus Grunow ---------------------- R 

ingens Rattray -------------------------------- R 

octonarius Ehrenberg --------------------------- R 

te11cllus (Brebisson) Andrews, n. comb ----------- R 

Hyalodiscus laePis Ehrenberg ----------------------- R 

tActinoptychus marylandicus Andrews, n. sp ----------- R 

senarius (Ehrenberg) Ehrenberg --------------- F 

* 'Virginicus (Grunow) Andrews, n. comb ----------- R 

':'Rattmyella i11conspiczw (Rattray) Hanna ____________ R 

tLiradiscus asperulus Andrews, n sp ----------------- R 

:j: ovalis Greville --------------------------------- R 

Biddulphia anrita var. obt11sa (Kiitzing) Hustedt ______ R 

* se'f'n.ici}~cularis (Brightwell) Boyel"" --------------- F 

tuomcyi (Bailey) Roper ------------------------- R 

tGoniothecium rogersii Ehrenberg -------------------- F 

':'Eucampia virginica Grunow ------------------------ R 

Terpsinoii americana (Bailey) Ralfs ----------------- R 

tT1·iceratium condecorwn Ehrenberg ------------------ F 

*Hemiaulus polymorphus var. virginica. Grunow -------- R 

tXantlziopy;ris microspinosa Andrews, n. sp ------------ R 

spp ------------------------------------------- R 

·;·Dossctia lzyalina Andrews, n. sp ---------------------- R 

tPeriptem tetracladia Ehrenberg --------------------- R 

':'Stephanogonia actinoptyclws (Ehrenberg) Grunow ____ R 

*Pseudopy:rilla america11a (Ehrenberg) Forti ---------- R 

Rhaplwncis amphiceros Ehrenberg ------------------- R 

* a.ugusta.ta. Pantocsek --------------------------- R 

t diamantella Andrews, n. sp --------------------- F 

* .Z.n1nnnzis Lohn1an ------------------------------- R 

'i~ lancettula Gru11ow ----------------------------- F 

':'Thalassionema obtusa (Grunow) Andrews, n. comb ---- R 

Tlwlassiothrix, longissima Cleve and Grunow __________ R 

Gmmmatophora angulosa Ehrenberg _________________ F 

marina (Lyngbye) Kiitzing ---------------------- R 

tCocconeis sp -------------------------------------- R 

Nauicula hcmzed!}i "'\V. Smith ------------------------ R 

lura Ehrenberg -------------------------------- R 

pc11nata Schmidt ------------------------------- F 

Diplo11eis crabro (Ehrenberg) Ehrenberg _____________ R 

':'Plcurosigma affine var. JJWJ'ylandica Grunow ---------- R 

·j-Am ph om sp ___________________ ------------------- R 

Stratigraphic si,qnificance.-Because the diatom 

a~semblage from the Calvert Formation reported by 

Lohman (1948) and the Choptank assemblage of 

this paper were studied by similar techniques and 

analyzed in a similar manner, a comparison of these 

two assemblages has some validity (table 1). The 

Choptank diatom assemblage contains substantially 

fewer taxa (60) than the Calvert assemblage (89). 

The difference may be due to the fact that the Cal-
DIATOM ASSEMBLAGE 7 

vert study was made on several stratigraphic horizons, whereas the Choptank study was made on a 

single horizon. However, from what I have observed 

of Calvert diatoms, the Calvert Formation appears 

to have a substantially more varied diatom assemblage. It contains, in good abundance, a considerable 

number of extinct species that do not occur in the 

Choptank Formation; many of these are known only 

from the Calvert Formation and its time-stratigraphic equivalents. The smaller percentage of extinct Miocene species in the Choptank Formation is 

balanced by the increased percentage of still-living 

taxa; this gives the Choptank assemblage a more 

modern character. (The figures in table 1 are reasonable considering the greater age of the Calvert 

Formation.) Nearly all these long-ranging forms 

also occur in the Calvert Formation, and a few 

range from Cretaceous or early Tertiary to modern 

seas. The increase in the Choptank in the number 

and percentage of extinct taxa ranging outside the 

Miocene probably reflects the improved knowledge 

of diatom ranges since 1948 more than anything 

else. 

TABLE !.-Comparison of diatom taxa from the Chesapeake 

Group 

Taxa 

New -----------------------

Extinct, known only from Miocene rocks -- Extinct, 1·anging outside Miocene 1·ocks ------------ Long mnging, still living in 

modern assemblages ------- Total ----------------

Chontank 

Formation, 

this report 

Number 

of Percent taxa 

11.7 

18 30.0 

10.0 

29 48.3 60 100.0 

Calvert 

Formation, 

Lohman (1948) 

Number of !'ercent taxa 

11 12.5 

37 41.5 

5.5 

36 40.5 89 100.0 

This Choptank diatom assemblage is significant 

in that it occurs in a formation dated, according to 

other fossil groups present, as middle Miocene. Because the age of the formation is relatively well 

known, this assemblage can be used with confidence 

in stratigraphic correlation. It is hoped that future 

investigation of older and younger Tertiary diatom 

assemblages in the Atlantic and Gulf Coastal Plains 

will so sharpen the precision of our knowledge of the 

stratigraphic ranges of diatom taxa that their 

usefulness for stratigraphic correlation will be 

greatly increased. 

Paleoecologic interpretation.-The paleoecology 

of the macrofaunas and microfossil faunas of the 

Choptank Formation has been discussed by Gernant 

(1970). The evidence from the diatom assemblage 

of stratigraphic zone 19 suggests a shallow neritic 

open-shelf marine depositional environment. Nearly 

all the still-living species in this fossil assemblage 

are reportedly common in coastal waters of modern 

temperate regions. Most of the diatoms in this assemblage are robust centric forms, which suggests 

that they were mainly planktonic but lived in shallow, agitated waters. The relative scarcity of benthonic forms indicates a muddy, shifting substratum, which is amply confirmed by the nature of the 

sediments. The diatom assemblage indicates nothing but normal marine salinity; not a single specimen of a nonmarine diatom was observed, not even 

salt-tolerant nonmarine forms. This confirms that 

the environment of growth and deposition was normal marine, far enough away from fresh-water 

sources to prevent the influx of nonmarine species. 

The relatively shallow-water environment is evident by contrasting this assemblage with those reported by Schrader (1973) and Koizumi (1973) 

from the deep-sea environments in the Pacific Ocean 

basin. Those assemblages are almost exclusively 

planktonic and contain many genera and species 

not found in the Choptank assemblage. The generally poor quality of the diatom assemblages in the 

Choptank Formation and the lack of diatoms in 

some strata are difficult to explain. The formation is 

generally considered to be entirely of shallow marine 

deposition, perhaps characterized by some fluctuation in depth of water but not by a drastic change 

in environment. At times clastic deposition might 

have increased to the point where diatoms were obscured by the influx of muddy sediments. It is also 

possible that diatoms were winnowed from the sedi-

, ments by currents or waves and carried away in 

i suspension to be deposited elsewhere. Perhaps at 

the time of deposition of the studied assemblage the 

waters were slightly deeper and a more open innershelf environment allowed a greater bloom of diatoms, which resulted in a more extensive and varied 

marine diatom assemblage. Preservation of diatoms in this type of fine sand-silt-clay lithology is 

often less than the best. The diatoms were initially 

deposited in soft muddy bottom sediments, but as 

a result of later compaction, many were crushed by 

pressure from adjacent sand grains in the matrix. 

This deformation may well account for the abundance of broken specimens, particularly of the larger 

forms, which is common in diatom assemblages from 

both the Calvert and the Choptank Formations. 

SYSTEi\IA TIC DESCRIPTIONS 

The classification of diatoms used in this report 

is based on a proposed classification by K. E. Lohman (unpub. data), and it is similar in many respects to the earlier classifications by Schutt (1896), 
8 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

Karsten (1928), and Hendey (1937). The treatment of the nomenclature in the generic and lower 

ranks is conservative, and the few changes made 

are in the direction of a less complex binomial system. It seems advisable to give a brief description 

of each taxon in the assemblage, for many of these 

species and varieties have been inadequately described, or the descriptive literature has long been 

out of print. The descriptions refer only to the specimens observed in this assemblage and do not necessarily indicate the total variation of the form; such 

information should be found in the general references cited in the synonymies. 

The first citation in each synonymy is to the 

original description of the taxon. The second citation 

includes references to the taxon in the literature in 

chronologie order using the nomenclature adopted 

throughout this report. Subsequent citations include 

synonyms, misidentifications, misspellings, and incorrect attributions of authorship. The synonymies 

are intended to be informative but not exhaustive, 

and further details should be sought in the general 

references. References not examined by the author 

are marked with an asterisk. Information on geologic ranges is taken mainly from Lohman (1948), 

but other sources are indicated. Comments on ecology of the living diatom species are mainly from 

Hustedt (1927-30, 1931-59, 1961-66). 

Kingdom PROTIST A 

Subkingdom PROTOCTIST A 

Division CHRYSOPHYT A 

Class DIA TOMOPHYCEAE 

Order CENTRALES 

Suborder DISCINEAE 

Family COSCINODISCACEAE 

Subfamily MELOSlROIDEAE 

Genus MELOSIRA Agardh, 1824 

Melosira westii Wm. Smith 

Plate 1, figures 1, 2 

Mclosim wcstii Wm. Smith, 1856, Synopsis of the British 

Diatomaceae, v. 2, p. 59, pl. 52, fig. 333. 

Hustedt, 1927, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 268-269, fig. 113. 

Hendey, 19G4, An introductory account of the smaller 

algae of British coastal waters, part V, Bacillariophyceae (Diatoms), p. 73, pl. 1, fig. 4; pl. 22, fig. 8. 

Description.-Valves disciform having heavy 

mantle which is rarely seen because of almost exclusive orientation in valve view. Diameter, about 

25,t-301t. Immediately inside n1antle rim is a row of 

very short, fine striae. The center of the valve is 

dominated by moderately large, irregularly distributed blobs of silica giving a patchy appearance 

which is often not sharply resolved by the light 

microscope. 

Remarks.-AJ elosira westi£ is common in this assemblage. 

Known geologic 1'ange.-From the Calvert Formation of Miocene age to coastal waters of modern 

oceans. 

Genus P ARALIA Heiberg, 1863 

Paralia complexa (Lohman) Andrews, n. comb. 

Plate 1, figures 3, 4 

Melosira complexa Lohman, 1948, Maryland Dept. Geology, 

Mines, and Water Resources Bull. 2, p. 156, pl. 5, fig. 

1-7. 

Descr;'iption.-Valves, circular, convex, diameter 

about 531t. Outer zone dominated by highly irregular 

but generally radiate striae, followed concentrically 

inward by a ring of concentric areolae, then by an 

inner concentric band, 3tt-5fL wide, of fine puncta, 

about 12 in 10fL, arranged in a quincuncial pattern. 

Center of valve highly convex with many irregularly 

distributed large puncta. 

Remarks.-Because of the affinity of this species 

for Paralia sulcata, it has been placed in the genus 

ParaHa following Boyer (1904) and Hendey (1964). 

Pa1·alia com,plexa is rare in this assemblage. 

Known geologt~c range.-The Calvert and Choptank Formation of Miocene age. 

Paralia sulcata (Ehrenberg) Cleve 

Plate 1, figures 5, 6 

Gaillonella sulcata Ehrenberg, 1838, Die Infusionsthierchen 

als volkommene Organismen, p. 170, pl. 21, fig. 5. * 

Paralia, sulcata (Ehrenberg) Cleve, 1873, Kgl. svenska akad. 

Handl., Bihang, v. 1, no. 13, p. 7. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 491, pl. 

135, fig. 9. 

Hendey, 1964, An introductory account of the smaller 

algae of British coastal waters, Part V, Bacillariophyceae (Diatoms), p. 73, pl. 23, fig. 5. 

Melosira s11lcata (Ehrenberg) Kiitzing, 1844, Die kieselschaligen Bacillarien oder Diatomeen, p. 55, pl. 2, fig. 7. 

Hustedt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 276-279, fig. 119. 

Lohman, 1941, U.S. Geol. Survey Prof. Paper 196, pt. 3, 

p. 64, pl. 12, fig. 1. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 156-157. 

Description.-Valve round, nearly flat, about 301t401t in diameter. Outer rim complexly loculate with 

an orderly arrangement of about 5 chambers in 101t. 

This is succeeded inwardly by a narrow band of fine 

puncta, about 14 in 10fL, arranged in a quincuncial 

pattern, then a narrow hyaline band less than 11t in 

width, afterwards a band of prominent radial striae, 

about 10 in 101t, which taper toward a hyaline center. 

Renzarks.-Pa.ralia, sulcata, is abundant and wellpreserved in this assemblage. 

Known geologic range.-Lohman (1948) states 

that the geologic range of this species covers all the 

Tertiary and that a variety has been reported from 
SYSTEMATIC DESCRIPTIONS 9 

the Cretaceous. It is common in modern marine 

coastal waters. 

Paralia sulcata var. coronata (Ehrenberg) Andrews, n. comb. 

Plate 1, figures 7, 8 

Gaillonella coronata Ehrenberg, 1845, Kgl. preuss. Akad. Wiss. 

Berlin Ber., p. 154. * 

Ehrenberg, 1854, Mikrogeologie, pl. 38, XXII, fig. 5. 

Melosira (Pamlia) sulcata var. coronata (Ehrenberg) Grunow in Van Heurck, 1882, Synopsis des Diatomees de 

Belgique, pl. 91, fig. 17. 

Melosira sulcata forma coronata Grunow. Hustedt, 1928, Die 

Kieselalgen Deutschlands, 6sterreichs, und der Schweiz, 

pt. 1, p. 278, fig. 119d. 

Description.-Valve round, nearly flat, about 23ft33ft in diameter. Outer rim complexly loculate similar to the type va.riety, succeeded inwardly by a 

hyaline band about 2ft in width, then by a band of 

somewhat knobby and irregular siliceous processes, 

3-4 in lOft, directed outwards from a broad, flat 

hyaline center. 

Remarks.-ParaUa s1.tlcata var. coronata is distinguished from the type variety by the lack of the 

quincuncial band of puncta and by the replacement 

of the radial striae by the knobby processes. 

This variety is abundant in the assemblage. 

Known geologic 1·ange.-The variety has been reported from the Calvert Formation of Miocene age, 

and Hustedt (1928) states that it occurs with the 

type variety in European coastal waters. 

Genus PYXIDICULA Ehrenberg, 1833 

Pyxidicula cruciata Ehrenberg 

Plate 1, figures 9, 10 

Pyxidicula cntciata Ehrenberg, 1843, Kgl. preuss. Akad. Wiss. 

Berlin, Phys. Abh., 1841, pl. 3, VII, fig. 6. 

Hustedt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 301-302. 

Dictyopyxis c1·uciata Ehrenberg, 1845, Kgl. preuss. Akad. 

Wiss. Berlin, Ber., 1844, p. 267. 

Ehrenberg, 1854, Mikrogeologie, pl. 33, XIII, fig. 7; XV, 

fig. 12; XVII, fig. 7. 

Steplzauopyxis turris var. arctica f. inamis Grunow, 1884, 

Kaisl.-Kgl. Akad. Wiss. Wien, Math.-Naturwiss. Kl., 

Denkschr. v. 48, p. 89, pl. 5, fig. 18, 21. 

Cleve-Euler, 1951, Kgl. svenska vetensk. akad. Handl. 

4th ser, v. 2, no. 1, p. 38, fig. 40 m, pl. 1, fig. 0. 

Descriptiou.-Valve round, markedly domed, 

probably in the shape of a flattened hemisphere 

although it was not observed in valve view. Diameter 

30p,-44ft, margin loculate, arching over as an areolate net arranged in an orderly hexagonal pattern. 

Large, round areolae, about 4 in lOv. Top and margin of valve free of spines or spine bases. 

Remarks.-Although often confused with Steplzanopy.ris. the genus Py.r1'd,icula is distinguished by 

its lack of prominent spines. 

The species is frequent in this assemblage. 

Known geologic range.-ln addition to its occurrence in the Choptank Formation, the species has 

been previously reported from the Calvert Formation. It is apparently restricted to fossil deposits 

that are probably all of Miocene age. 

Genus STEPHANOPYXIS Ehrenberg, 1844 

Stephanopyxis corona (Ehrenberg) Grunow 

Plate 1, figures 11, 12 

Systephania corona Ehrenberg, 1845, Kgl. preuss. Akad. Wiss. 

Berlin, Ber., 1844, p. 272. 

Ehrenberg, 1854, Mikrogeologie, pl. 33, XV, fig. 22. 

Steplzanopy:r:is corona (Ehrenberg) Grunow in Van Heurck, 

1882, Synopsis des Diatomees de Belgique, pl. 83 ter, 

fig. 10, 11. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 490-491, 

pl. 135, fig. 13. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 157. 

Description.-Valve round, convex, probably in 

the shape of a hemisphere markedly flattened in 

the center. Diameter 53ft-74ft. Margin loculate, arching over as an areolate net arranged in an orderly 

hexagonal pattern. Areolae, 3-4 in l01t, large, hexagonal. A ring of prominent thornlike siliceous spines 

radiate from a zone near the margin, but they do 

not extend beyond the margin. A second ring of 

prominent siliceous spines occurs at about half the 

distance from center to margin. 

Remarks.-The species is rare in this assemblage. 

Known geologic 1·ange.-This species is reported 

only fron1 the Calvert and Choptank Formations of 

Miocene age. 

Stephanopyxis lineata (Ehrenberg) Forti 

Plate 1, figures 13, 14 

Stephanodiscus? lineatus ( =Peristcplzania lineata) Ehrenberg, 1854, Mikrogeologie, pl. 33, XIII, fig. 22. 

Stephanopyxis lineata (Ehrenberg) Forti, 1912, La Nuova 

Notarisia, p. 83.* 

Forti, 1913, R. ist. veneto sci., Atti, v. 72. pt. 2, p. 1547, 

pl. 11, fig. 21, 23; pl. 12, fig. 3. * 

Hanna, 1932, California Acad. Sci. Proc., 4th ser., v. 20, 

p. 219, pl. 16, fig. 9-11. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

ResourcPs Bull. 2, p. 158, pl. 6, fig. 5, 

Description.-Valve round, nearly flat, 40/t-721t in 

diameter. Entire surface of valve shows a generally 

regular, hexagonal, areolate net. Areolae, about 4 1,~ 

in 10/'• large, hexagonal. A ring of thornlike siliceous 

spines, somewhat irregularly spaced, occurs 2v-3v 

from the outer margin. 

Rernarks.-This species resembles Cosct'nod1~scus 

lineatus, but the large thornlike spines near the 

periphery are sufficient to distinguish it from that 

species. 

Steplwnopyxis lineata is frequent in this assemblage. 
10 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

Known geolog-ic range.-Koizumi (1973, fig. 10) 

states that S. hneata ranges from foraminiferal 

zones N. 9 toN. 15 of Blow (1969), that is, throughout the middle Miocene. 

Stephanopyxis turris (Greville and Arnott) Ralfs 

Plate 2, figures 1, 2 

Oreswellia tur1·is Greville and Arnott, 1857, Royal Soc. Edinburgh Trans., v. 21, pt. 4, p. 64, pl. 6, fig. 109. * 

Stephanopyxis tun·is (Greville and Arnott) Ralfs in Pritchard, 1861, History of the Infusoria, 4th ed., p. 826, 

pl. 5, fig. 74. 

Hustedt, 1928, Die Kieselalgen Deutschlands. 6sterreichs, 

und der Schweiz, pt. 1, p. 304-307, fig. 140. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

Resources Bull. 2, p. 158. 

Desc'ription.-Valve round, highly domed, so that 

the height is greater than the diameter; the usual 

orientation is the girdle view. Valve with flat or 

slightly concave base, expanded to a slight flange at 

base, then tapering to nearly parallel sides, arching 

over on top as a rounded or flattened dome. Diameter, 25tt-30tt. Surface of valve covered with a 

somewhat irregular hexagonal areolate net, with 

4-6 hexagonal areolae in lOtt. Bases of large siliceous 

spines on the domed end are sometimes observed. 

Remarks.-The species is rare in this assemblage. 

Known geologic 1·ange.-This is a long-ranging 

species, extending from Cretaceous to modern oceans 

(Lohman, 1948). According to Hustedt (1928) it is 

a pelagic form frequently occurring in modern marine deposits. 

Subfamily COSCINODISCOIDEAE 

Genus COSCINODISCUS Ehrenberg, 1838 

Coscinodiscus apiculatus Ehrenberg 

Plate 2, figure 3 

Coscinodiscus apiculatus Ehrenberg, 1844, Kgl. preuss. Akad. 

Wiss. Berlin, Ber., 1844. p. 77. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 43. 

Hustedt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 449-452, fig. 248. 

Hanna, 1932, California Acad. Sci. Proc., 4th ser., v. 20, 

p. 178-179, pl. 6, fig. 1. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

Resources Bull. 2, p. 158. 

Description.-Valve round, flat, diameter of observed specimen, 1171t. Surface covered with large 

round areolae, about 31 ~ in 101t, uniform in size, arranged primarily in radial lines and secondarily 

in a curved decussating pattern. Areolae somewhat 

less closely packed near center, and there is a small 

central hyaline area. 

Remarks.-Although this species may be confused 

with Coscinodiscus perforatus, it lacks the fine terminal pores of the striae of that species. 

Cosc,inodiscus apiculatus is rare in this assemblage. 

Known geologic range.-Miocene (Calvert Formation and equivalent) to modern oceans. 

Coscinodiscus curvatulus Grunow 

Plate 2, figure 4 

Coscinodisc11s curvatulus Grunow in Schmidt, 1878, Atlas 

der Diatomaceenkunde, pl. 57, fig. 33. 

Hustedt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 406-410, fig. 214. 

Lohman, 1941, U.S. Geol. Survey Prof. Paper 196, part 

3, p. 74, pl. 15, fig. 8. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 160. 

Description.-Valve round, flat, diameter of observed specimen, 48tt. Valve divided into curved 

sectors, more or less 11 in number, each defined by 

a curved row of areolae extending from the margin 

to the center, with the remainder of the sector filled 

by curved parallel rows of areolae, these rows decreasing in length toward the margin of the adjacent 

sector. About 7 round areolae in lOtt, arranged complexly in a curved radial as well as decussating pattern. This ordered pattern extends to the center of 

the valve. 

Renwrks.-Coscinodiscus curvatulus is rare in 

this assemblage. 

Known geologic range.-This species is found 

throughout the Tertiary and is a pelagic form frequently occurring in modern oceans. 

Coscinodiscus inclusus Rattray 

Plate 2, figure 5 

Coscinodiscus hzclusus Rattray, 1889, Royal Soc. Edinburgh 

Proc., v. 16, p. 482. 

Lohman, 1938, U.S. Geol. Survey Prof. Paper 189-C, p. 

86, pl. 20, fig. 3. 

Description.-Valve round, somewhat convex with 

narrow, indistinctly striated margin. Diameter of 

observed specimen, 381t. Valve divided into straight 

sectors, more or less seven in number, each defined 

by a straight row of areolae extending from the 

margin to the center, with the remainder of the 

sector filled by parallel rows of areolae, these rows 

decreasing in length toward the margin of the 

adjacent sector. About 7-8 rounded areolae in lOp, 

arranged primarily in straight radial lines but also 

secondarily in a curved decnssating pattern. Center 

of valve more or less closely packed with areolae. 

Remarks.-The species is rare in this assemblage. 

Known geologic range.-The species has only been 

reported previously from the Calvert Formation of 

Miocene age at Richmond, Va., and the Etchegoin 

Formation of Pliocene age of California (Lohman, 

1938). 
SYSTEMATIC DESCRIPTIONS 11 

Coscinodiscus marginatus Ehrenberg 

Plate 2, figures 6, 7 

Coscinodiscus marginatus Ehrenberg, 1843, Kgl. preuss. Akad. 

Wiss. Berlin, Phys. Abh. 1841, p. 329, 371. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 44; pl. 33, 

XII, fig. 13; pl. 38, XXII, fig. 8. 

Hustedt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 416-418, fig. 223. 

Lohman, 1941, U.S. Geol. Survey Prof. Paper 196, part 3, 

p. 71-72, pl. 14, fig. 1, 6. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 162, pl. 7, fig. 4. 

Description.-Valve round, slightly convex, dian1-

eter 441t-9lp. Marginal rim zone 61I-8ft wide, composed of radial striae, about 6 in lOv. Inside of rim 

zone is an areolate net composed of hexagonal areolae, about 31 ·~ in 101!, with a secondary orientation 

in curved decussating rows. Areolae fill the valve 

surface by intercalation of progressively shorter 

radial rows toward the margin. The areolate net is 

somewhat less than perfectly regular, and the center 

of the valve is closely packed with areolae. 

Remarks.-A variant of this species that has 

round areolae rather than a tightly packed areolate 

net has been reported from the Calvert Formation. 

This form, although not observed in this assemblage, 

n1ay also occur in the Choptank Formation. 

Coscinodiscus uzarpinatus is rare in this assemblage. 

Known geolo,rric range.-Reported from the Moreno Shale of Cretaceous and Paleocene age of California by Long, Fuge, and Smith (1946). It is still 

living in modern seas. 

Coscinodiscus oculus-iridis Ehrenberg 

Plate 2, figure 8 

Cosci11odiscus ocl(lus-iriclis Ehrenberg, 1841, Kgl. preuss. 

Akad. Wiss. Berlin, Abh., 1839, p. 147. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 42; pl. 19, 

fig. 2. 

Hustedt, 1!.128, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 454-456, fig. 252. 

Hanna, 1£132, California Acad. Sci. Proc., 4th ser., v. 

20, p. 183, pl. 9, fig. 4. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 1G:3. 

Descriptioll.-Valve round, flat, large, diameter of 

observed specimen, 1961,. Marginal rim zone about 

8tt wide composed of short striae, about 4-5 in 101,. 

Inside of rim zone is an areolate net with a primary 

radial orientation superimposed on a secondary 

orientation of curved clecussating rows. Areolae fill 

the valve surface by intercalation of progressively 

shorter radial rows toward the valve margin. Areolae, 2-3 in 1011, somewhat coarser near the center 

than near the margin. The center of the valve has 

a rosette of elongate radiating areolae. 

Remarks.-This species rarely occurs in fragments large enough for identification. There is a considerable amount of fragmentary debris in the assemblage, much of it from large disks such as C. 

oculus-iridis. 

Known geologic range.-Entire Tertiary to 

n1odern oceans (Lohman, 1948). 

Coscinodiscus perforatus Ehrenberg 

Plate 2, figure 9 

Coscinodiscus perfomtus Ehrenberg, 1844, Kgl. preuss. Akad. 

Wiss. Berlin, Ber., 1844, p. 78. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 46. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 506, 

pl. 135, fig. 2. 

H ustedt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 445-449, fig. 245. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

Resources Bull. 2, p. 163. 

Descyiption.-Valve round, flat, diameter of observed specimen, 102~t. Valve surface covered with 

an areolate net having a primary radial orientation 

superimposed on a secondary orientation in curved 

decussating rows. Areolae fill the valve surface by 

intercalation of progressively shorter radial rows 

toward the valve margin. The definitive characteristic of the species is that each short row ends in a 

single small punctum. Hexagonal areolae, about 

4~,~ in 101t, somewhat finer near the margin, net less 

than perfectly organized. Center of valve may be 

completely filled with areolae or show a small 

hyaline area. 

Remarks.-The species is rare in this assemblage. 

Known geologic range.-From the Calvert Formation of Miocene age to modern seas. 

Coscinodiscus perforatus var. cellulosa Grunow 

Plate 2, figure 10 

Coscinodiscns perforatus var. cellulosa Grunow, 1884, Kaisl.-

Kgl. Akad. Wiss. Wien, Math.-Naturwiss. Kl., Denkschr., v. 48, p. 75. 

Hustedt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 447, fig. 246. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources, Bull. 2, p. 163-164, pl. 8, fig. 3. 

Description.-Valve round, flat, diameter of observed specimen, 8lv. The variety cellulosa is similar 

to the type variety, except that the areolae are 

round rather than closely packed into a hexagonal 

shape. Areolae, about 6 in lOv, finer near the margin. 

Remarks.-This variety is rare in the assemblage. 

Knozl'n geologic range.-From the Calvert Formation of Miocene age to modern seas. A reported 

occurrence of this variety by Rattray in the upper 

Eocene of Oamaru, New Zealand, appears to be 

erroneous. 
12 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

Coscinodiscus perforatus var. pavillardi (Forti) Hustedt 

Plate 2, figures 11, 12 

Coscinodiscus pavillardi Forti, 1922, R. Comit. Thalassograf. 

Ital. Mem., 97, p. 124, pl. 8, fig. 143. * 

Coscinodiscus perforatus var. pavillardi (Forti) Hustedt, 1928 

Die Kieselalgen Deutschlands, 6sterreichs, und de; 

Schweiz, pt. 1, p. 447-448, fig. 247. 

Description.-Valve round, flat, diameter 59f-t140ft. Similar to the type variety except that some 

marginal areolae are round but areolae near the 

center are more closely packed into hexagons. The 

short striae terminate in a single punctum near the 

margins, but the striae of intermediate length terminate without the punctum. Areolae, about 3 in 

lO,u, finer near the margin. 

Re1narks.-This variety is somewhat more frequent than the type variety or var. celhllosa, but it 

is still rare in the assemblage. 

Known geologic Yange.-This assemblage to 

modern seas. 

Coscinodiscus radiatus Ehrenberg 

Plate 2, figure 13 

Coscinodisc11s mdiatus Ehrenberg, 1841, Kgl. preuss. Akad. 

Wiss. Berlin, Abh., 1839, p. 148, pl. 3, fig. lA-C. 

Hustedt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz pt. 1, p. 420-421, fig. 225. 

Lohman, 1941, U.S. Geol. Survey Prof. Paper 196, part 

3, p. 73, pl. 14, fig. 7, 8. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 164. 

Desc1·iption.-Valve round, flat, diameter of observed specimen, 1001,. Valve surface covered with 

an areolate net having a primary radial orientation 

and a secondary orientation in curved decussating 

rows. Areolae fill the valve surface by intercalation 

of progressively shorter rows toward the margin. 

Areolae round, 3-4 in 101t, somewhat finer near the 

n1argin. There is often a small hyaline area in the 

center of the valve. 

Remm·ks.-This species is similar in appearance 

to C. perforatus but lacks the terminal puncta on the 

shorter rows. In the figured specimen the widely 

spaced marginal dots are artifacts and not marginal 

apiculi. 

Coscinodiscus radiatus is rare in this assemblage. 

Knoum geologic range.-Miocene to modern 

oceans. 

Coscinodiscus rothii (Ehrenberg) Grunow 

Plate 3, figures 1, 2 

Hcterostcplwnia 1·othii Ehrenberg, 1854, Mikrogeologie, pl. 

35A, XIII B, fig. 4, 5. 

Cosci11odiscus rothii (Ehrenberg) Grunow in Schneider, 1878, 

Naturw. Beitr. zu Kenntn. der Kaukasuslander, p. 

125*. 

Husteclt, 1928, Die Kieselalgen Deutschlancls, 6sterreichs, 

und der Schweiz, pt. 1, p. 400-406, fig. 211. 

Coscinodiscus subtilis Ehrenberg, 1843, Kgl. preuss. Akad. 

Wiss. Berlin, Phys. Abh., 1841, p. 412. 

Ehrenberg, 1844, Kgl. preuss. Akad. Wiss, Berlin, Ber., 

1844, p. 78. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 35; pl. 33, 

XIV, fig. 7. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 506. 

DescripUon.-Valve round, tapering upward from 

margin to a raised annular ring, but depressed in 

center. Diameter 58fL-92,u. Surface of valve divided 

into many sectors, each of which is defined by a 

relatively straight row of areolae from margin to 

center, then by progressively shorter rows to edge 

of adjacent sector, Areolae distinct and round, about 

7 in lO,u, with primary radial and secondary curved 

decussating orientations. Areolate net not rigidly 

organized. The center may contain a small hyaline 

ring enclosing a few areolae. 

Re1narks.-Hustedt (1928, p. 404-406) gives a 

lengthy discussion of the reasons for abandoning 

the ill-defined Coscinodiscus subtilis in favor of C. 

1·othii, and his conclusions are followed here. Reference to the occurrence of C. subtilis in the Chesapeake Group are included in the above synonymy. 

Coscinodiscus Yothii is rare in this assemblage. 

Known geologic range.-Calvert Formation of 

Miocene age to modern seas, where it is common. 

Some of the varieties range into brackish and even 

fresh-water environments. 

Coscinodiscus vetustissimus Pantocsek 

Plate 3, figure 3 

Coscinodiscus vctustissimus Pantocsek, 1886, Beitrage zur 

Kenntniss der fossilen Bacillarien Ungarns, pt. 1, p. 

71, pl. 20, fig. 186. 

Husteclt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 412-414, fig. 220. 

Lohman, 1938, U.S. Geol. Survey Prof. Paper 189-C, p. 

86, pl. 20, fig. 7. 

Description.-Valve round, flat, diameters of observed specimens, 581t and 67v. Surface of valve 

divided into many ill-defined sectors, each limited 

by a relatively straight row of areolae from margin 

to center, then by progressively shorter parallel rows 

to edge of adjacent sector. Areolae hexagonal, closely 

packed, about 5 in 1011, having a primary radial 

orientation and a much less well-defined secondary 

curved decussating orientation. The center contains 

a small cluster of areolae surrounded by a hyaline 

ring. 

Rema1·k.s.-This species is rare in the assemblage. 

Kno1cn geologic range.-Originally described by 

Pantocsek (1886) from the Miocene of Hungary; 

reported by Lohn1an ( 1938) from the Pliocene of 

California. Koizumi (1973, fig. 10) states that this 

species ranges from foraminiferal zones N. 9 to N. 
SYSTEMATIC DESCRIPTIONS 13 

17 of Blow (1969), that is, throughout middle Miocene into late Miocene. 

Genus CRASPEDODISCUS Ehrenberg, 1845 

Craspedodiscus coscinodiscus Ehrenberg 

Plate 3, figure 4 

Craspedodiscus coscinodiscus Ehrenberg, 1845, Kgl. preuss. 

Akad. Wiss. Berlin, Ber., 1844, p. 266. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 108; pl. 33, 

XV, fig. 8; pl. 33, XVI, fig. 8. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 500-

501, pl. 135, fig. 3. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 166, pl. 8, fig. 6. 

Craspedodiscus microdiscus Ehrenberg, 1854, Mikrogeologie, 

pl. 33, XVII, fig. 4. 

Description.-Valve round, having nearly flat 

outer flange and sharply convex center, diameter 

741-l-1001-l. Narrow zone of marginal striations, about 

6 in lOft, followed by a broad flange about half the 

diameter of valve in some specimens. Surface of 

flange is covered with a net of radiating round 

areolae, about 3 in 101t. Center of valve contains a 

dome with markings distinct from those of the outer 

flange, the rows of puncta, about 5 in 101t, having 

a primary radial orientation and a secondary orientation in curved decussating rows. There is a small 

hyaline area at the center of the valve. 

Remarks.-The species is rare in this assemblage. 

Known geologic range.-Previously reported only 

from the Calvert Formation of Miocene age of the 

eastern United States. 

Genus ACTINOCYCLUS Ehrenberg, 1838 

Actinocyclus ellipticus Grunow 

Plate 3, figures 5, 6 

Actinocyclus ellipticus Grunow in Van Heurck, 1883, Synopsis 

des Diatomees de Belgique, pl. 124, fig. 10. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 502, pl. 

134, fig. 1. 

Hustedt, 1929, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 533-534, fig. 303. 

Description.-Valve broadly elliptical, but slightly 

tapered toward bluntly rounded ends. Length 271,-

791-l. Margin of valve irregularly striated, about 9 

striae in 101t. Marginal border band of fine puncta, 

about 13 rows in lOv, arranged both in radial rows 

and in scalloped decussating curved rows. Irregular 

radial rows of coarser puncta, about 7 in 101t along 

the row, extend toward the center of the valve, 

where similar coarse puncta are more or less randomly arranged. Pseudonodulus located near inner 

edge of the zone of fine puncta, on the side of the 

valve, but off center. 

Rerna1'ks.-The species is rare in this deposit. 

Known geologic range.-Although Peragallo and 

Peragallo (1902, p. 418, pl. 104, fig. 8) state that 

this species occurs today in the Mediterranean, I 

think that this is an error. Koizumi (1973, fig. 10) 

states that A. ellipticus ranges from foraminiferal 

zones N. 9 toN. 15 of Blow (1969), that is, throughout the middle Miocene. 

Actinocyclus ingens Rattray 

Plate 3, figure 10 

Actinocyclzcs ingcns Rattray, 1890, Jour. Queckett Microscop. 

Club, 2d. ser., v. 4, p. 149-150, pl. 11, fig. 7. 

Kanaya, 1959, Tohoku Univ. Sci. Repts., 2d ser. (Geology), v. 30, p. 97-99, pl. 7, fig. 6-9; pl. 8, fig. 1-4. 

Desc1·iption.-Valve round, having sharp marginal 

convexity but otherwise nearly flat. Narrow marginal rim with fine striae, about 18 in 101-l. This rim 

is succeeded inwardly by a ring of fine puncta, about 

18 in lOp", arranged in a quincuncial pattern. Main 

part of valve covered with radial rows of areolae, 

about 5-6 areolae in lOf-t, more closely spaced near 

the margin than the center. Areolae near margin 

show slight tendency toward a secondary arrangement in curved decussating rows, are arranged radially in midpart of valve and more or less randomly 

near the center. They are basically round, but sometimes show slight angularity because of lateral compression. Center of valve is an irregular hyaline 

area. Pseudonodulus obscure in observed specimen. 

Remarks.-This is a most difficult species to identify because of the long confusion in the literature 

as to its affinities for Actinocyclus or Coscinodiscus. 

Kanaya (1959) has ably discussed this problem after 

examination of many specimens from California 

and Japan. The single specimen which I have observed in the Choptank Formation has much greater 

affinity for Actinocyclus and hence I have placed 

it in Actinocyclus ingens rather than in Coschwdiscus ele[Jans. I have not been able to identify a pseudonodulus in the Choptank specimen. Apparently a 

pseudonodulus multiplex-a cluster of minute foramina-occurs in this species, but this is difficult to 

resolve under the light microscope. 

The relationship of Actinocyclus ingens to A. 

oculatus Jouse and A. oclwtensis Jouse seems problematical at this time. In fact, a careful analysis and 

comparison of Actinocyclus ingens, A. oclwtensis, 

A. tsuga ntensis Kanaya, and Coscinodiscus elegans 

would be helpful in understanding the relationships 

between these taxa. The Choptank specimen of 

Actinocyclus ingens seems to be very similar to 

forms published as A. ochotensis Jouse. The species 

does not seem to have been previously reported 

from the Atlantic Ocean basin, but is known from 

California, Japan, and central Europe (Kanaya, 

1959, p. 98). 

Actinocyclus ingens is rare in this assemblage. 
14 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

Known geologic 1·ange.-Perhaps restricted to the 

Miocene. If the species is eventually proved indistinguishable from A. ochote1~sis J ouse, however, the 

range must be extended to the Holocene. 

Actinocyclus octonarius Ehrenberg 

Plate 3, figure 7 

A.ctinocyclas octonarius Ehrenberg, 1838, Die Infusionsthierchen als vollkommene Organismen, p. 172, pl. 21, fig. 7.'~ 

Hendey, 1937, Discovery Reports, v. 16, p. 262. 

Lohman, 1941, U.S. Geol. Survey Prof. Paper 196, part 

3, p. 77, pl. 16, fig. 4. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Mineral Resources Bull. 2, p. 167, pl. 8, fig. 8. 

A.ctinocyclus ehrenbergii Ralfs in Pritchard, 1861, History of 

Infusoria, 4th ed., p. 834. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 502. 

Hustedt, 1929, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 525-533, fig. 298. 

Desc1·iption.-Valve round, large, nearly flat, thin 

and fragile, usually found only as fragments. Diameter 46,tt-114fL. Marginal zone of fine puncta, 16-

20 in 10fL, arranged in a quincuncial pattern. Most of 

the surface of the valve is divided into sectors by 

single radial rows of puncta extending from the center to the marginal zone. Each sector is filled with 

parallel striae composed of discrete puncta, arranged 

normal to the margin of sector, but not parallel to 

the dividing striae. Each sector is filled by progressively shorter striae on both sides of the sector. 

About 10 striae in 10fL; about 7 puncta in 10,tt along 

the striae. Pseudonodulus on inner edge of the quincuncial zone. 

Remarlts.-The justification for using Actinocyclus octonarius in place of A; elz ren bergii has been 

well presented by Hendey (1937) and Lohn1an 

( 1942) and need not be repeated here. 

The species is rare in this deposit. 

Known geologic range.-Miocene to modern seas 

in which its occurrence is widespread. 

Actinocyclus tenellus (Brebisson) Andrews, n. comb. 

Plate 3, figures 8, 9 

Eupodisc1ts tenellus Brebisson, 1854, Mem. Soc. Imp. Sci. 

Nat. Cherbourg, v. 2, p. 257, pl. 1, fig. 9.':' 

Actinocycl1ts moniliformis Ralfs in Pritchard, 1861, History 

of Infusoria, 4th ed., p. 834. 

Grunow in Van Heurck, 1883, Synopsis des Diatomees de 

Belgique, p. 124, fig. 9. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 502-

503, pl. 134, fig. 2. 

Actinocyclus ehrenbel'gii var. tenella (Brebisson). Hustedt, 

1929, Die Kieselalgen Deutschlands, 6sterreichs, und 

der Schweiz, pt. 1, p. 530, fig. 302. 

Description.-Valve round, nearly flat. Diameter 

561t-701t. Margin with fine striations, about 18 in 

101t, followed inwardly by a narrow quincuncial zone 

of fine puncta, about 16 in 101t. The greater part of 

valve is divided into sectors by radial punctate 

striae, and the sectors are filled with punctate striae 

in an arrangement identical to that of Actinocyclus 

octonarius. 10 striae in 10fL; 6 puncta in 10fL along 

the striae. Pseudonodulus at inner margin of quincuncial zone. There is often an irregular hyaline 

area at the center of the valve. 

Remarks.-This species shows obvious affinities 

for Actinocyclus octonarius, and Hustedt (1929) 

makes it a variety of that species. In this assemblage, however, the specimens are distinctive from 

that species in the n1ore robust markings of the 

valve and the generally smaller size. 

Knou•n geologic range.-Miocene to modern 

oceans. 

Genus HYALODISCUS Ehrenberg, 1845 

Hyalodiscus laevis Ehrenberg 

Plate 4, figures 1, 2 

Hyalodiscus laevis Ehrenberg, 1845, Kgl. preuss. Akad. Wiss. 

Berlin, Ber., 1845, p. 78. * 

Ehrenberg, 1854, Mikrogeologie, pl. 33, XV, fig. 17. 

Husteclt, 1928, Die Kieselalgen Deutschlands, 6sterreichs, 

uncl der Schweiz, pt. 1, p. 294-295, fig. 135. 

Description.-Valve round, markedly convex, 

351t-531t in diameter. Valve slopes upward fron1 

n1argin to raised umbilical ring at about half the 

radius; center of valve depressed. Extreme margin 

of valve has very narrow zone of striations, about 

10 in 101t. Ren1ainder of valve irregularly mottled, 

and fine structure, if any, not observed. Top of umbilical ring irregularly scattered with sharp, welldefined puncta in some specimens. 

Remarks.-Hyalodi.scus laevis is rare in this assemblage. 

Knozcn geologic ranpc.-Miocene to modern coastal marine and brackish waters. 

Family ACTINODISCACEAE 

Subfamilv ACTINOPTYCHOIDEAE 

Genus ACTINOPTYCHUS Ehrenberg, 1843 

Actinoptychus marylandicus Andrews, n. sp. 

Plate 4, figures 3-6 

Diamwsis.-Valve round, flat, fragile. Diameter 

5"1,u-95p. Valve margin often missing or poorly presenrecl, main part of valve divided into sectors, 14-

18 in the specin1ens observed. Sectors are alternately 

elevated and depressed, the elevated group having 

different n1arkings from the depressed group. (1) 

Coarsely punctate sectors (pl. 4, figs. 3, 5) : single 

row of puncta at margin, succeeded inwardly by a 

narrow but distinct hyaline band; main part of sector covered with parallel rows of puncta, about 10 

in 10 ~, arranged in a quincuneial pattern, but showing a blaclelike hyaline area extending one-third to 

one-half the length of the sector from the hyaline 
SYSTEMATIC DESCRIPTIONS 15 

central area; bladelike hyaline area caused by absence of 1-3 rows of puncta in center of sector. (2) 

Finely punctate sectors (pl. 4, figs. 4, 6) : each sector shows a single spinelike process, centrally located 

near the margin ; n1ain part of sector covered with 

fine puncta, about 16 in lOp,, and arranged in a hexagonal pattern. There is no bladelike hyaline area 

analogous to that of the alternate sectors. Central 

area hyaline, vaguely marked, about one-fifth of 

the diameter of the valve, warped near n1argin to 

join alternate sectors on different planes. 

Remarks.-This species is very similar to Actinoptych us ?)irginicus, except that the hyaline areas in 

the coarsely punctate sectors are bladelike rather 

than irregular. It is perhaps similar to A. vulgaris 

var. australis Grunow, but the lack of an adequate 

description of that form makes comparison difficult. 

Actinoptyclz us marylandicus is rare in this assemblage. 

Known geologic rwzge.-Middle Miocene) not previously reported. This may be a short-ranging species and hence useful as a marker fossil. 

Holotype: USGS diatom catalog No. 3904-12 (pl. 

4, figs. 3, 4), diameter 541t. Paratype: USGS diatom 

catalog No. 3901-28 (pl. 4, figs. 5, 6), diameter 591t. 

Actinoptychus senarius (Ehrenberg) Ehrenberg 

Plate 4, figures 7, 8 

Acti11ucyclus :;cJwrius Ehrenberg, 1838, Die Infusionsthierchen als vollkommene Organismen, p. 172, pl. 21, fig. 

6.::: 

Actiuoptydzus sc1wrius (Ehrenberg) Ehrenberg, 1843, Kgl. 

preuss. Akacl. Wiss. Berlin, Phys. Abh., 1841, p. 400, 

pl. 1, fig. 27. 

Hencley, 1937, Discovery Repol'ls, v. 16, p. 271-272. 

Lohman, 1941, U.S. Geol. Survey Prof. Paper 196, part 

3, p. 80-81, pl. 1G, fig. 9. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 170. 

Actinopf!fclz us wzdlllahcs Kiitzing. Boyer, 1904, Maryland 

Geol. Survey, Miocene, p. 499-500, pl. 134, fig. 4. 

ActinopfJJclllrs zwdulat/ls (Bailey) Ralfs. Hustedt, 1929, Die 

Kieselalgen Deutschlands, 6sterreichs, und der Schweiz, 

pt. 1, p. 475-4 78, fig. 264. 

Description.-Valve round, fiat; diameter 411t661'. Valve divided into six sectors, alternately elevated and depressed, one group having different 

markings fron1 the other group. ( 1) Coarsely punctate sectors (pl. 4, fig. 7) : surface completely and 

regularly covered with parallel rows of puncta, 

about 14 in 101,, arranged in a quincuncial pattern. 

This layer appears to be underlain by a vague net 

of larger rectilinear areolae, about 4 in 101t. (2) 

Finely punctate sectors (pl. 4, fig. 8). Each sector 

shows a single outwardly directed tubular process 

near the center of the margin, some extending 

slightly beyond the edge of the valve; surface completely and regularly covered with fine puncta, about 

17 in lOp,, arranged in a hexagonal areolate net. This 

appears to be underlain by a vague net of larger 

hexagonal areolae, about 4 in lOp,. Central area hyaline, vaguely marked, about one-fifth the width of 

the valve, warped near margins to join alternate 

sectors on different planes. 

Rerna,rk~s.-The nomenclature of this species is 

ably discussed by Hendey (1937) and Lohman 

(1942) and need not be repeated here. 

The species occurs frequently in this assemblage. 

Known geologic range.-Actino]Jtych'us senar·ius 

is reported from the Cretaceous of California to cool 

coastal waters of modern seas, where it is common. 

Adinoptychus virginicus (Grunow) Andrews, n. comb. 

Plate 4, figures 9-12 

Actinoptyclws vulgaris var. vil'ginica Grunow in Van Heurck, 

1883, Synopsis des Diatomees de Belgique, pl. 121, 

fig. 7. 

Actinoptychus vulgaris var. 'Uirginiae Grunow. Boyer, 1904, 

Maryland Geol. Survey, Miocene, p. 500. 

Description.-Valve round, fiat, fragile, diameter 

35/t-721.1. Valve margin often missing or poorly preserved, n1ain part of valve divided into sectors, 

10-18 in the specimens observed. Sectors are alternately elevated and depressed, one group having different markings from the other group. (1) Coarsely 

punctate sectors (pl. 4, figs. 9, 11), single row of 

puncta at margin, succeeded inwardly by a narrow 

but distinct hyaline band; main part of sector 

covered with parallel rows of puncta, about 10 in 

101,, arranged in a quincuncial pattern, but showing 

an irregular hyaline area in the center of each sector, seemingly an extension of the hyaline central 

area of the valve. (2) Finely punctate sectors (pl. 

4, figs. 11, 12), some specimens show a single 

spinelike process, centrally located near the margin; 

main part of sector covered with fine puncta, about 

16 in 101t, arranged in a hexagonal pattern and completely filling the sector. Central area hyaline, 

vaguely ·marked, about one-fifth of the diameter of 

the valve, warped near margin to join the alternate 

sectors on different planes. 

Remarks.-This species is very similar to Actinoptychus marylandicns, differing only in the irregular hyaline spaces on the coarsely punctate 

sectors. In view of the doubtful validity of A. vulgaris, as expressed by Hustedt (1929, p. 482), I have 

raised Grunow's variety to specific rank. 

Actinoptyclws 'Vir,qinicus is rare in this assemblage. 

Known geologic range.-Calvert and Choptank 

Formations of middle Miocene age. 
16 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

Suborder EUPODISCINEAE 

Family EUPODISCACEAE 

Subfamily EUPODISCOIDEAE 

Genus RA TTRA YELLA Hanna, 1932 

Rattrayella inconspicua (Rattray) Hanna 

Plate 5, figures 1, 2 

Eupodiscus inconspicuus Rattray, 1888, Jour. Roy. Microscopical Soc., v. 9, p. 911.':' 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 498, pl. 

135, fig. 6, 7. 

Rattray ella hwonspicua (Rattray) Hanna, 1932, California 

Acad. Sci. Proc., 4th ser., v. 20, p. 210-211, pl. 14, fig. 

5; pl. 15, fig. 1, 2. 

DescripUon.-Valve round, convex, diameter 7211.-

88/l. Six short tubular processes extend above valve 

surface at regular intervals along the margin. Surface of valve covered with a ~et of hexagonal areolae, about 3 in lOft and variable in size. The net is 

not rigidly organized in rows of areolae. 

Renwrks.-The number of marginal tubular processes may vary, and they are not always easy to discern. The specimens figured by Hanna (1932) show 

a more obvious pattern of curved decussating rows 

in the areolate net than my specimens show. 

The species is rare in this deposit. 

Known geologic range.-Miocene from both eastern and western United States. 

Suborder AULISCINEAE 

Family LIRADISCACEAE 

Subfamily LIRADISCOIDEAE 

Genus LIRADISCUS Greville, 1865 

Liradiscus asperulus Andrews, n. sp. 

Plate 5, figures 3-5 

D,iagnos,is .-Valve ovate, con vex, observed length 

23fl-43fl, width 13rt-15fl. Single row of prominent 

thornlike spines along margin of valve, extending 

normal to the plane of the valve. Entire valve surface covered with an irregular network of anastomosing low ridges, a few of these near the center 

developed into irregular rugose processes. 

Remarks.-This species is distinguished by the 

marginal spines and the rugose processes irregularly 

distributed on the center of the valve. 

Liradiscus asperulus is rare in this assemblage. 

Known ueologic ran.ge.-Middle Miocene. Not previously reported. 

Holotype: USGS diatom catalog No. 3903-7 (pl. 

5, fig. 3), length 401'". Para type: USGS diatom catalog No. 3904-6 (pl. 5, fig. 4), length 43fl. Para type: 

USGS diatom catalog No. 3902-14 (pl. 5, fig. 5), 

length 23p. 

Liradiscus ovalis Greville 

Plate 5, figures 6, 7 

Liradiscus ovalis Greville, 1865, Trans. Microscop. Soc. London, v. 13, n. s., p. 5, pl. 1, fig. 15, 16.* 

Rattray, 1889, Roy. Soc. Edinburgh Proc., v. 16, p. 670-

671. 

DescTiption.-Valve convex, ovate with bluntly 

pointed apices. Surface of valve covered randomly 

with short spines. A network of low ridges connects 

adjacent spines. Length of observed specimens 22/l30/l. 

Remarks.-Liradiscus ovalis is rare in this assemblage. 

Known geologic range.-Upper Eocene (Oamaru) 

to middle Miocene (this assemblage). Rattray described the species from Barbados. 

Order BIDDULPHIALES 

· Suborder BIDDULPHIINEAE 

Family BIDDULPHIACEAE 

Subfamily BIDDULPHIOIDEAE 

Genus BIDDULPHIA Gray, 1831 

Biddulphia aurita var. obtusa (Kiitzing) Hustedt 

Plate 5, figure 8 

Odontella obtusa Kiitzing, 1844, Die kieselschaligen Bacillarien oder Diatomeen, p. 137, pl. 18, fig. VIII, 1-3, 6-8. 

Biddulphia aurita var. obtusa (Kiitzing) Hustedt, 1930, Die 

Kieselalgen Deutschlands, 6sterreichs, und der Schweiz, 

pt. 1, p. 848-849, fig. 502. 

Biddulphia roperiana Greville, 1859, Quart. Jour. Microscop. 

Sci., v. 7, p. 163, pl. 8, fig. 11-13. * 

Grunow in Van Heurck, 1882, Synopsis des Diatomees de 

Belgique, pl. 99, fig. 4-6. 

Description.-Valve convex, subrhomboidal, with 

bluntly pointed apices. Length of observed specimen, 47 p. Surface of valve covered with radiating 

rows of puncta, about 7 in 1 Ott, showing a secondary 

pattern of curved decussating rows. Two short 

tubular processes, about 4fl in diameter, extend vertically from the plane of the valves at the apices. 

Rema1·ks.-This variety is rare in this assemblage. 

Known geologic range.-Middle Miocene to coastal 

waters of modern seas. 

Biddulphia semicircularis (Brightwell) Boyer 

Plate 5, figures 9, 10 

Triceratimn semici?·cula,re Brightwell, 1853, Quart. Jour. 

Royal Microscop. Soc., v. 1, p. 252, pl. 4, fig. 21. * 

Bidclulphia semicircularis (Brightwell) Boyer, 1901, Acad. Nat. 

Sci. Philadelphia Proc., v. 52, p. 726. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 494, 

pl. 134, fig. 10. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources Bull. 2, p. 173-174. 

Tricemtimn obtusurn Ehrenberg, 1854 (in part), Mikrogeologie, pl. 18, fig. 49. 

Euodia brightwelli Ralfs in Pritchard, 1861, History of Infusoria, 4th ed., p. 852. 

Grunow in Van Heurck, 1883, Synopsis des diatomees de 

Belgique, pl. 126, fig. 20. 

Descripl'ion.-Valve lunate, convex near margins, 

but nearly flat on top. Convex side is highly arched, 

concave side slightly arched to nearly straight. The 

two apices are more or less protracted in some specimens. Surface of valve covered with roughly circular 
SYSTEMATIC DESCRIPTIONS 17 

rows of large discrete puncta, 4-7 in lOft, concentric 

around a hyaline central area. Length of observed 

specimens, 45ft-70ft. 

Remarks.-Biddulphia swmicircular·is has a robust 

valve, which is generally well preserved. It is frequent in this assemblage. · 

Known geologic range.-Middle Miocene, occurring in the Calvert and Choptank Formations. 

Biddulphia tuomeyi (Bailey) Roper 

Plate 5, figure 11 

Zygoce1·os Tuomeyi Bailey, 1844, Am. Jour. Sci., v. 46, p. 138, 

pl. 3, fig. 3-9. 

Biddulphia Tuomeyi (Bailey) Roper, 1859, Royal Microscopical Soc. Trans., v. 7, p. 8, pl. 1, fig. 1, 2.* 

Grunow in Van Heurck, 1882, Synopsis des Diatomees de 

Belgique, pl. 98, fig. 2-3. 

Hustedt, 1930, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 834-836, fig. 491. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources, Bull. 2, p. 17 4. 

Biddulphia t1·idens Ehrenberg, 1840, Kgl. Akad. Wiss. Berlin, 

Ber., p. 205. * 

Boyer, 1901, Acad. Nat. Sci. Philadelphia Proc., v. 52, 

p. 695. 

Description.-Valve usually oriented in girdle 

view, length of observed specimens, 48ft-57ft· Ba.s~e 

slightly extended beyond main part of valve, which 

consists of a large domed central process and two 

somewhat deflected tubular apical processes." Tips 

of apical processes are usually missing. Main part of 

valve covered with large discrete puncta, arranged 

radially on the central dome but apparently of ran- ' 

dom distribution elsewhere. 

Remarks.-~he illustrations of Hustedt (1930) 

and Van Heurck (1882) show two pronounced 

lateral swellings or domes between the central dome 

and the apical processes. There are only faint suggestions of these in specimens observed in this assemblage. 

This species is rare in this assemblage. 

Known geologic range.-Cretaceous to Holocene 

according to Lohman (1948). 

Genus GONIOTHECIUM Ehrenberg 1843 

Goniothecium rogersii Ehrenberg 

Plate 5, figures 12, 13 

Goniotheciz.on rogersii Ehrenberg, 1843, Kgl. Akad. Wiss. Berlin, Phys. Abh., 1841, p. 401, 416. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 92, 93. 

Hanna, 1932, California Acad. Sci. Proc., 4th ser., v. 20, 

p. 192, pl. 11, fig. 4-6. 

Lohman, 1948, Maryland Dept. Geology, Mines, and 

Water Resources, Bull. 2, p. 179. 

Descrription.-Valve irregularly elliptical, convex, 

48ft-80ft in length. Valve has a tiered "weddingcake" structure with three levels of surface elevation. These levels are defined by circular "escarpments," which are truncated by the lateral margins 

of the valves. Valve surface covered by large scattered puncta having no discernible orientation. 

Rwma1·ks.-Goniotheciu.1n TogeTsii is a distinctive 

species, usually well preserved, and it is frequent in 

this assemblage. 

Known geologic Yange.-Middle Miocene of eastern and western United States to San Joaquin Formation of Pliocene age of California. 

Family EUCAMPIACEAE 

Genus EUCAMPIA Ehrenberg, 1839 

Eucampia virginica Grunow 

Plate 5, figures 14, 15 

Eucampia virginica Grunow in Van Heurck, 1882, Synopsis 

des Diatomees de Belgique, pl. 95 bis, fig. 6. 

Eucam.pia zoodiacus Ehrenberg (in part). Boyer, 1927, Acad. 

Nat. Sci. Philadelphia, Proc., v. 78, suppl., p. 116. 

Hustedt, 1930, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 772. 

Descript·ion.-Valve elongate-elliptical with bluntly rounded apices, length of observed specimens, 

62t-t-76t-t. Surface of valve with irregular rows of 

puncta, about 8 in lOft, radiating from center. Central area small with a single process of undetermined 

nature. Ends of valve hyaline, with no trace of 

projecting processes. 

Remarks.-There seems to be little justification 

for lumping this species with E. zoodiacus in view 

of the former's coarser punctation and lack of projecting apical processes. 

Euca-rnpia virginica is rare in this assemblage. 

Known geolog·ic rang e.-Middle Miocene (Calvert 

and Choptank Formations) . 

Family TERPSINOACEAE 

Genus TERPSINOE Ehrenberg, 1843 

T erpsinoe americana (Bailey) Ralfs 

Plate 5, figures 16, 17 

Tetragrwmma americana Bailey, 1854, Smithsonian Contributions to Knowledge, v. 7, art. 2, p. 7-8, fig. 1. 

Terpsinoc americana (Bailey) Ralfs in Pritchard, Infusoria, 

p. 859. 

Boyer, 1927, Acad. Nat. Sci. Philadelphia Proc., v. 78, 

suppl., p. 145. 

Hustedt, 1930, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 1, p. 9·00-901, fig. 541. 

Description.-Valve elongate, markedly undulating, divided by two transverse septa into three expanded segments ; ends rounded, distinct from adjacent segments, but not separated from them by 

septa. Length 6l1.L-90ft· Surface of valve covered with 

large but vague puncta in a randomly oriented pattern. In many specimens, central part of valve shows 

one or more slitlike processes of undetermined nature. Ringlike hyaline areas at apices appear to be 

bases for some sort of attachment process, but do 

not pro.i ect far above the surface. 
18 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

Rema1'ks.-'I'he species is rare in this assemblage. 

Known geologic 1·ange.-Miocene to modern coastal marine and brackish waters. 

Suborder TRICERA TIINEAE 

Family TRICERATACEAE 

Genus TRICERA TIUM Ehrenberg, 1841 

Triceratium condecorum Ehrenberg 

Plate 5, figures 18, 19 

Tricemtiwn condecoJ·um Ehrenberg, 1845, Kgl. preuss. Akad. 

Wiss. Berlin, Ber., 1844, p. 272. 

Grunow in Schmidt, 1882, Atlas der Diatomaceenkunde, 

pl. 76, fig. 28. 

Hanna, 1932, California Acad. Sci. Proc., 4th ser., v. 20, 

p. 221, pl. 17, fig. 1, 3. 

Hustedt, 1959, Schmidt's Atlas der Diatomaceenkunde, 

pl. 4 78, fig. 14-21 

Biddulphia condecora (Ehrenberg) Boyer, 1901, Acad. Nat. 

Sci. Philadelphia Proc., v. 52, p. 720. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 492-

493, pl. 134, fig. 7. 

Triceratium interpunctaturn Grunow in Schmidt, 1882, Atlas 

der Diatomaceenkunde, pl. 76, fig. 7. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

Resources Bull. 2, p. 175, pl. 10, fig. 2. 

Biddulphia hLte-rpunctaturn (Grunow) Boyer, 1900, Acad. Nat. 

Sci. Philadelphia Proc., v. 52, p. 720. 

Boyer, 1904, Maryland Geol. Survey, Miocene, p. 494, pl. 

134, fig. 9. 

Description.-Valve triangular, has a raised rim 

and depressed triangular center. Sides vary from 

slightly concave to straight to slightly convex. 

Apices vary from obtusely rounded to a rounded 

point. Length of side, 441t-60fL. Surface of valve 

covered with a net of areolae, arranged primarily 

in a radial pattern and secondarily in a roughly concentric pattern around the center. About 5 areolae 

in 10,u near center, somewhat more closely spaced 

at the margin. Radial striae terminate in a single 

small punctum, in a manner analogous to that of the 

short striae of Cosdnodiscus pet'/01·atus. A small 

area of finer areolae occurs on each of the apices. 

Re,marks.-I have seen no convincing evidence in 

any of the literature cited in the synonymy on how 

Trice1·atium hderpunctatum differs from T. condecorutn. As they occur in the same deposits and appear to be indistinguishable, T. interpunctatmn has 

been placed in synonymy and T. condecormn preserved on the basis of priority. 

Tn'cerathon condecoru,m is frequent in this assemblage. 

Known geologic l'ange.-Hustedt reports this species from the upper Eocene of Oamaru, New Zealand. It is well known from the Calvert and Choptank Formations of middle Miocene age. Koizumi 

(1973, fig. 10) states that T. condecorutn ranges 

from foraminiferal zones N. 9 to N. 17 of Blow 

(1969), that is, from middle to late Miocene. 

Suborder HEMIAULIINEAE 

Family HEMIAULACEAE 

Genus HEMIAULUS Ehrenberg, 1845 

Hemiaulus polymorphus var. virginica Grunow 

Hemiaulus polyrnorphus var. virginica Grunow, 1884, Kais1.-

Kgl. Akad. Wiss. Wien, Math.-Naturwiss. Kl., Denkschr., v. 48, p. 66, pl. B, fig. 46. 

Description.-Valve usually oriented in girdle 

view, length of observed specimen about 33,u. Main 

part of valve divided into three bulbous parts, of 

which the center is largest and best defined. Ends 

of valve have tubular processes, about 301t in length, 

slightly deflected outward; a small terminal spine 

on inside margin of tubular processes deflected inward. Entire valve covered with well defined puncta 

having a random distribution. 

Remarks.-The form observed in this assemblage 

is identical to that figured by Grunow (1884) as var. 

virginica in that it is also characterized by the three 

bulbous processes, is the same size, and has the 

same length-height ratio. Whether or not these features are consistent enough to maintain the validity 

of this taxon remains to be determined. 

Known geologic J'ange.-The variety has been 

previously reported only by Grunow ( 1884) from 

the Calvert Formation of Miocene age at Richn1ond, 

Va. Lohman (1948) states that the species ranges 

from Cretaceous to Holocene. Koizumi (1973) states 

that the species ranges from foraminiferal zones N. 

9 to N. 15 of Blow (1969), that is, throughout the 

middle Miocene. 

Order HETERALES 

Suborder HETERINEAE 

Family XANTHIOPYXACEAE 

Genus XANTHIOPYXIS Ehrenberg, 1845 

Xanthiopyxis microspinosa Andrews, n. sp. 

Plate 6, figures 1-3 

Diagnosis.-Valve ovate, convex, with blunt, obtusely pointed apices. Length 32fL-52,u; width 21fL32fL. Margin of valve narrow, striated, about 12 

striae in 10fL. Entire valve surface covered with 

short fine spines having a random distribution. 

Remarks.-The species is distinguished by its external shape and by the randomly and unevenly distributed short spines over the entire surface. 

Xant111:opyxis micTospinosa is rare in this assemblage. 

Known geologic rang e.-Middle Miocene. Not previously reported. 

Holotype: USGS diatom catalog No. 3901-43 (pl. 

6, fig. 1), length, 52fL. Para type: USGS diatom catalog No. 3772-37 (pl. 6, fig. 2), length, 32fL. Paratype: USGS diatom catalog No. 3900-46, (pl. 6, 

fig. 3) length, 32fL. 
SYSTEMATIC DESCRIPTIONS 19 

Xanthiopyxis spp. 

A large number of varied forms of Xanthiopy:ris 

were observed among the diatoms from locality 

6098. In general, these specimens were small (less 

than 50ft in maximum dimension). They varied in 

outline from round through ovate to elliptical to 

elongate. The ornamentation varied from hyaline to 

punctate to spinose, with both puncta and spines 

differing in size and distribution. In general, only 

one specimen of each type was observed, and there 

was no real evidence to determine if each represented a valid species or simply unique individuals. 

Because of the rarity of each form in the sample 

and hence their dubious value in stratigraphic age 

determination, it seems sufficient to mention their 

occurrence but to forgo a taxonomic analysis. 

Genus DOSSETIA Azpeitia, 1911 

Dossetia hyalina Andrews, n. sp. 

Plate 6, figures 4-7 

Diagnosis.-Valve ovate, highly convex, surrounded at the n1argin with a vaguely laminate flange 

parallel to the plane of the valves, strongly dentate 

on its outer n1argin. Observed length, 37 rt-50rt, but 

probably somewhat greater because the tips of the 

larger marginal spines are usually broken off. The 

convex n1ain part of the valve is hyaline and shows 

no discernible structure with the light microscope 

under an oil-immersion objective. 

Remarks.-This species is similar to Dossetia 

temperei Azpeitia except that the valve is hyaline 

rather than granular. 

Dossetia hyalina. is rare in this assemblage. 

Known geologic range.-MidcUe Miocene. Not previously reported. 

Holotype: USGS diatom catalog No. 3901-38 (pl. 

6, fig. 4), length, 45p. Para type: USGS diatom catalog No. 3901-19 (pl. 6, fig. 5), length, 37 p. Paratype: USGS diatom catalog No. 3901-29 (pl. 6, 

fig. 6), length, 40v. Para type: USGS diatom catalog 

No. 3904-3 (pl. 6, fig. 7), length, 42ft. 

Genus PERIPTERA Ehrenberg, 1845 

Periptera tetracladia Ehrenberg 

Plate 5, figures 20, 21 

Pcriptcra tctmcladia Ehrenberg, 18--15, Kgl. preuss. Akad. Wiss. 

Berlin, Ber., 184--1, p. 270. 

Ehrenberg, 1854, Mikrogeologie, pl. 33, XVIII, fig. 9. 

Grunow in Van Heurck, 1882, Synopsis des Diatomees 

de Belgique, pl. 83 ter, fig. 7-9. 

Hanna, 1932, California Acad. Sci. Proc., 4th ser., v. 20, 

p. 205, pl. 13, fig 8. 

Lohman, 19-!8, Maryland Dept. Geology, Mines, and Water 

Resources Bull. 2, p. 178-179. 

Description.-Valve roughly quadrangular in 

girdle view, slightly swollen at center. Length of 

observed specimens, 28rt-32rt. Prominent large thornlike spines, many of which are partially broken, extend from the valve margin normal to the plane of 

the valves. One or two rows of fine puncta, about 12 

in lOrt but irregular in distribution, occur around the 

girdle. 

Ren~a1·ks.-Periptera tetracladia is rare in this 

assemblage. 

Known geologic range.-Middle Miocene to Pliocene according to Lohman (1948). 

Genus STEPHANOGONIA Ehrenberg, 1845 

Stephanogonia aclinoptychus (Ehrenberg) Grunow 

Plate 6, figure 8 

Mastogonia actinoptychus Ehrenberg, 1845, Kgl. preuss. Akad. 

Wiss. Berlin, Ber., 1844, p. 269. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 109; pl. 33, 

XIII, fig. 16. 

Stephanogonia actz~noptychus (Ehrenberg) Grunow in Van 

Heurck, 1882, Synopsis des Diatomees de Belgique, pl. 

83 ter, fig. 2-4. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

Resources Bull. 2, p. 178. 

Stephanogonia polyacantha Forti, 1913, R. ist. veneto sci., 

Atti, v. 72, pt. 2, p. 1560, pl. 12, fig. 11.* 

Hanna, 1932, California Acad. Sci. Proc, 4th ser., v. 20, 

p. 218, pl. 16, fig. 8. 

Descn:ption.-Valve broadly ovate, convex, length 

of observed specimen, 62rt; width, 48p,. Narrow marginal rim, main part of valve divided into more or 

less equal sectors by irregular septa which fuse near 

the center into an irregular ring. Valve surface 

covered with randomly scattered puncta, fine but 

distinct. 

Remnrks.-The species is rare in this assemblage. 

Known geologic rang e.-Reported only from Miocene assemblages in eastern and western United 

States. 

Familv PYXILLACEAE 

Genus PSEUDOPYXILLA Forti 1909 

Pseudopyxilla americana (Ehrenberg) Forti 

Plate 6, figures 9-12 

Rhizo.sole11ia americana. Ehrenberg, 1843, Kgl. preuss. Akad. 

Wiss. Berlin Abh. 1841, p. 422. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 98; pl. 33, 

fig. 13/20, 17/14. 

Pseudopy.rilla. americana (Ehrenberg) Forti, 1909, La Nuova 

Notarisia, ser. 20, pl. 1, fig. 6, 7.* 

Pyxilla. americana (Ehrenberg) Grunow in Van Heurck, 1882, 

Synopsis des Diatomees de Belgique, pl. 83 bis, fig. 1-3. 

Description.-Only tips of valves preserved, two 

general shapes: (1) round, tapering gently to a 

truncated point; (2) round, curved, tapering rather 

abruptly to an end bearing one or two sharply deflected spines. Ends of valves hyaline, with a sharp 

boundary between punctate main part of valve. 

Puncta fine, about 12 in lOrt, in a closely spaced 
20 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

random distribution. A single transverse septum is 

seen in some valve tips. 

Remarks.-Although the gently and abruptly 

tapered tips are somewhat different in appearance, 

both have been customarily included in this s1pecies. 

P. americana is rare in this assemblage. 

Known geologic range.-This species has been reported only from the middle Miocene of eastern and 

western United States. 

Order PENNALES 

Suborder ARAPHIDINEAE 

Fa·mily FRAGILARIACEAE 

Subfamily FRAGILARIOIDEAE 

Genus RHAPHONEIS Ehrenberg, 1845 

Rhaphoneis amphiceros Ehrenberg 

Plate 6, figures 13, 14 

Rhaphoneis mnphiceros Ehrenberg, 1844, Kgl. preuss. Akad. 

Wiss. Berlin, Ber., 1844, p. 87. 

Ehrenberg, 1854, Mikrogeologie, pl. 33, XIV, fig. 22. 

Van Heurck, 1885, Synopsis des Diatomees de Belgique, 

p, 147; 1881, pl. 36, fig. 22, 23. 

Hustedt, 1931, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 2, p. 174-176, fig. 680. 

Hanna, 1932, California Acad. Sci. Proc., 4th ser., v. 20, 

p. 211-212, pl. 15, fig. 3-5. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

Resources Bull. 2, p. 180. 

Description.-Valve elliptical-lanceolate, nearly 

flat, with produced, sharply rounded apices. Length 

of observed specimens, 45fL-71fL, width about 221t. 

Surface of valve covered with curved radial rows 

of distinct puncta, 7-8 rows in lOp. and 7-8 puncta 

in 101-L along the row. In addition to a primary arrangement as curved transverse striae, the puncta 

show a secondary arrangement in nearly straight 

longitudinal rows. Pseudoraphe very narrow near 

apices, but slightly expanded near center. Transverse striae may or may not show continuity across 

the pseudoraphe. 

Remar'ks.-The species is rare in this assemblage. 

Known geologic range.-Miocene to modern coastal marine and brackish waters. 

Rhaphoneis angustata Pantocsek 

Pia te 7, figures 1, 2 

Rhaphoneis angustata Pantocsek, 1886, Beitdige zur Kenntnis 

der fossilen Bacillarien Ungarns, pt. 1, p. 33, pl. 11, 

fig. 97; pl. 30, fig. 313. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

Resources, Bull. 2, p. 180-181, pl. 11, fig. 11. 

Description.-Valve elongate, tapering slightly to 

bluntly rounded apices. Length of observed specimens, 28fL-58fL, width about 711 .• Striae short, 8-9 

rows in 10f-L, composed of only 2 or 3 longitudinal 

rows of puncta. The striae are parallel in the center, 

becoming slightly radial near the apices, and finer 

striae curve completely around the ends of the 

valves. Pseudoraphe comparatively wide for the 

narrow valve, about one-quarter to one-fifth of the 

width. Pairs of transverse striae may or may not 

be alined across the pseudoraphe. 

Remarks.-The species is rare in this assemblage. 

Known geologic range.-Rhaphoneis angustata 

has been previously reported only from the Miocene 

of Hungary and from the Calvert Formation of 

Miocene age. 

Rhaphoneis diamantella Andrews, n. sp. 

Plate 6, figures 15-18 

Diagnosis.-Valve lozenge-shaped, obtuse to 

rounded at the center of the lateral margins, tapering by straight or slightly concave sides to bluntly 

pointed apices. Length, 331-L-701-L width, 15f-L-22JL. 

Transverse striae composed of large pearllike 

puncta, about 5 in 101-L in both transverse and longitudinal directions. In addition to a primary orientation in slightly radiating transverse rows, the 

puncta are secondarily oriented in slightly curved 

longitudinal rows. Pseudoraphe very narrow near 

the apices but slightly expanded in the central region. Individual transverse striae are consistently 

staggered at, and do not show continuity across, the 

pseudoraphe. 

Remarks.-This species resembles R. gemmifera 

in coarseness of the pearllike puncta ~and in their 

general arrangement. It differs significantly in outline, however, showing a much more nea.rly perfect 

lozenge shape, sharper medial .angulation on the 

lateral margins, and less protracted apices. Comparison of R. dia·mantella with specimens of R. gemrnifera under the scanning electron microscope confirms this differentiation. R. diamantella shows 

clean, round openings which form the puncta and 

only vestigial fine pore fields in the apical regions, 

whereas R. gemmifera shows openings with paired, 

inward-projecting apiculi, and well-defined fine pore 

fields near the apices. The presence of inward-directed apiculi in the large pores has been cited as a 

generic characteristic of Rhaphonei:;;. However, 

these structures do not seem to occur in either R. 

diamaniella or R. lancettula as viewed under the 

scanning electron microscope, and hence must be 

characteristic of only so.me species of Rhaphoneis. 

Known geologic range.-Middle Miocene. Known 

only from this assemblage. This distinctive species 

may be an excellent index fossil if it proves to be 

of wide distribution. 

Holotype: USGS diatom catalo~g No. 3772-29 (pl. 

6, fig. 17), length, 62f-L. Para type: USGS diatom catalog No. 3771-9 (pl. 6, fig. 15), length, 401-L. Paratype: 

USGS diatom catalog No. 3772-52 (pl. 6, fig. 16), 
SYSTEMATIC DESCRIPTIONS 21 

length, 50ft. Para type: USGS diatom c~atalog No. 

3900-14 (pl. 6, fig. 18), length, 70ft. 

Rhaphoneis immunis Lohman 

Plate 7, figure 3 

Rhaphoneis immunis Lohman, 1948, Maryland,-Dept. Geology, 

Mines, and Water Resources Bull. 2, p. 182, pl. 11, fig. 6. 

Description.-Valve laneeolate, tapering to narrow rounded apices. Length about 45ft, width about 

6ft. Single row of marginal puncta, 6-7 puncta in 

lOft. Inner row of larger puncta, 5lfi puncta in lOf!, 

parallel to the marginal row, but with distribution 

bearing no obvious relationship to the marginal 

puncta. Pseudo raphe hyaline in center, tapering 

~apidly toward the apices and becoming obscured 

by the converging rows of inner, coarser puncta. 

Remarks.-Although the species as here observed 

is somewhat smaller and the marginal puncta are 

a bit more closely spaced than those described by 

Lohman (1948, p. 182), it agrees with Lohman's 

species completely in the unusual dual punctation of 

the valves. 

Rhaphoneis imrnunis is rare in this assemblage. 

Known geologic 1·ange.-The Calvert and Choptank Formations of Miocene age. 

Rhaphoneis lancettula Grunow 

Plate 7, figures 4, 5 

Rhaphoneis lancettula Grunow in Pantocsek, 1886, Beitdige 

zur Kenntnis der fossilen Bacillarien Ungarns, p. 35, 

pl. 27, fig. 271. 

FragUaria a.mphiceros Ehrenberg, 1854, Mikrogeologie, pl. 18, 

fi~ 77. 

Desc'ription.-Valve narrow, elongate, lateral margins tapering gently toward the apices, then becoming nearly parallel to attenuate the valve in the 

apical direction. Apices rounded, slightly capitate 

in some specimens. Length, 55f!-76f!; width, 7-81;~p .. 

Puncta, 6 in lOf!, large, evenly spaced, and arranged 

in regular rows both transversely and longitudinally. 

Pseudoraphe narrow, identified only as the central 

hyaline space between longitudinal rows of puncta, 

and not noticeably wider than the spaces between 

other rows of puncta. 

Re1narks.-This species is recognized by the narrow v·alve with the attenuate apices and by the 

rectilinear arrangement of the puncta. The regular 

arrangement of the puncta as well as the very narrow pseudoraphe relegate the pseudoraphe in this 

species to more of a concept than a reality. 

The species occurs frequently in this assemblage. 

Known geolog1~c range.-Choptank Formation at 

this locality; described by Grunow from the Miocene 

deposit at Richmond, Va., probably from the Calvert 

Formation. 

Genus THALASSIONEMA Grunow, 1881 

Thalassionema obtusa (Grunow) Andrews, n. comb. 

Plate 7, figures 6-8 

Thalassiothrix? nitzschioides var. obtusa Grunow in an Heurck, 

1881, Synopsis des Diatomees de Belgique, pl. 43, fig. 6. 

Description.-Valve small, lanceolate, with rounded apices. Length, 32ft-37ft; width, about 5ft-6ft. 

Along the lateral margins of the valve is a row of 

single puncta or very short transverse striae, about 

91 ~ in lOft. Pseudoraphe relatively wide, lanceolate, 

hyaline. 

Remarks.-This form see·ms to be sufficiently distinct from Thalassionema nitzschioides to be recognized as a separate sp·ecies. 

Thalassionerna obtusa is rare in this assemblage. 

Known geologic range.-Previously reported only 

from the Rappahannock deposit of middle Miocene 

age by Grunow in Van Heurck, (1881). 

Genus THALASSIOTHRIX Cleve and Grunow, 1880 

Thalassiothrix longissima Cleve and Grunow 

Plate 7, figures 9, 10 

Synedra. thalassiothrix Cleve, 1873, Kgl. SV'enska vetensk. akad. 

Handl., Bihang, Bd. 1, No. 13, p. 22, pl. 4, fig. 24. 

TlwlassiothriJ· longissima Cleve and Grunow, 1880, Kgl. svenska VEtensk, akad. Handl., v. 17, No. 2, p. 108. 

Hustedt, 1932, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 2, p. 24 7, fig. 726. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water 

Resources Bull. 2, p. 185. 

Descriph~on.-Valve extremely elongate, narrow, 

with bluntly rounded apices. Length unknown in this 

assemblage as only fragments were observed; width, 

3ft-4ft. Transverse striae short, marginal, about 12 

in lOft. Pseudoraphe about one-third the width of 

the valve, narrowing near the apices. 

Rentarks.-Although this species should properly 

be named Thalassiothri:r tlwlassiothri:r (Cleve) 

Cleve and Grunow, a change seems pointless after 

over 90 years' consistent use of T. longZ:ssirna. 

Thalassioth ri.r long,issirna, is rare in this assemblage. 

Known geologic range.-Middle Miocene to modern seas, where it is especially abundant in cold 

waters. 

Subfamily T ABELLARIOIDEAE 

Genus GRAMMATOPHORA Ehrenberg,1839 

Grammatophora angulosa Ehrenberg 

Plate 7, figures 11-13 

Grammatophom mzgulosa Ehrenberg, 1841, Kgl. preuss. Akad. 

Wiss. Berlin, Abh., 1839, p. 153. 

Ehrenberg, 1854, Mikrogeologie, pl. 18, fig. 88. 

Hustedt, 1931, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 2, p. 39-40, fig. 564. 

Description.-Valve elongate-elliptical with bluntly rounded apices, length 27 ft-62~-t. Only the more 
22 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

robust girdle part was preserved in observed specimens, so the fine structure is not known from this 

assemblage. Girdle view shows prominent S-shaped 

internal septa of the valve (see pl. 7, fig. 13). 

Rema1·ks.-The species is frequent in this assemblage. 

Known geologic 1·ange.-Middle Miocene to coastal 

waters of modern seas, where it is common. 

Grammatophora marina (Lyngbye) Kiitzing 

Plate 7, figures 14, 15 

Diatoma marinum Lyngbye, 1819, Hydrophyt. dan., pl. 62A. * 

Grammatophora marina (Lyngbye) Kiitzing, 1844, Die kieselschaligen Bacillarien oder Diatomeen, p. 128, pl. 17, 

fig. XXIV, 1-6, pl. 18, fig. I, 1-5. 

Van Heurck, 1881, Synposis des Diatomees del Belgique, pl. 

53, fig. 10-13. 

Hustedt, 1931, Die Kieselalgen Deutschlands, Osterreichs, 

und der Schweiz, pl. 2, p. 43-44, fig. 569. 

Desc~ription.-Valve elongate-elliptical with bluntly rounded apices, length of observed specimens, 

38,u-44,u. Fine structure not preserved, and only the 

more robust internal septa are known from this assemblage. Septa nearly flat, not sharply bent as in 

G1·ammatophoTa angulosa. Single elliptical central 

opening is the most striking feature of the valve. 

Remarks.-The species is rare in this assemblage. 

Known geologic range.-Middle Miocene to modern marine coastal waters, where it is common. 

Suborder MONORAPHIDINEAE 

Family ACHNANTHACEAE 

Subfamilv COCCONEOIDEAE 

Genus COCCONEIS Ehrenberg, 1838 

Cocconeis sp. 

Pia te 7, figure 16 

Descript'ion.-Valve broadly elliptical with bluntly rounded apices, the one measurable 41,u in length 

and 28p- in width. Marginal puncta, about 14 in 

lO,u, about 9-10 rows in 10,u of large, somewhat 

irregularly spaced puncta in the lateral areas. Pseudoraphe narrow, bounded on either side by rows of 

large puncta. 

Remarks.-This form is seen rarely in the assemblage, but only as fragments of the pseudoraphe 

valve. The figured specimen is the most nearly complete valve observed. This appears to be a distinctive species which may have stratigraphic value, but 

formal diagnosis must await the discovery of the 

raphe valve as well as better specimens of the pseudoraphe valve. 

Known geologic range.-This assemblage. 

Suborder BIRAPHIDINEAE 

Family NAVICULACEAE 

Subfamily NA VICULOIDEAE 

Genus NAVICULA Bory, 1822 

Navicula hennedyi W. Smith 

Plate 7, figures 17, 18 

Navicula hennedyi W. Smith, 1856, Synopsis of the British 

Diatomaceae, v. 2, p. 93. 

Hendey, 1964, An introductory account of smaller algae 

of British coastal waters, part 5, Bacillariophyceae 

(Diatoms), p. 212-213, pl. 33, fig. 14. 

Hustedt, 1964, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 3, p. 453-455, fig. 1516. 

Description.-Valve broadly elliptical with obtusely rounded apices. Length of observed specimens, 48,u-58,u; width 23,u-28,u. Axial area narrow. 

Transverse striae divided into two bands: (1) the 

axial striae forming a narrow band, 1-4 puncta wide 

on either side of the raphe and having a parallel 

orientation; (2) the marginal striae forming a 

broader band which narrows toward the apices and 

has a radiate orientation. Transverse striae, about 

11 in 10,u, distinctly punctate. Between the axial and 

marginal striae are broad half-lanceolate hyaline 

lateral areas. 

Remarks.-Navicula hennedy1: is rare in this assemblage and occurs mostly as broken fragments 

of valves. 

Known geologic range.-Miocene to modern marine coastal waters, where it is common. 

Navicula lyra Ehrenberg 

Plate 7, figure 19 

Navicula lym Ehrenberg, 1843, Kgl. preuss. Akad. Wiss. Berlin, Phys. Abh., 1841, p. 419, pl. 1, I, fig. 9a. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water Resources Bull. 2, p. 185-186. 

Hendey, 1964, An introductory account of smaller algae 

of British coastal waters, Part V, Bacillariophyceae 

(Diatoms), p. 209, pl. 33, fig. 2. 

Hustedt, 1964, Die Kieselalgen Deutr~hlands, 6sterreichs, 

und der Schweiz, pt. 3, p. 500-502, fig. 1548a. 

Description.-Valve elliptical-lanceolate with 

slightly produced, subrostrate, narrowly rounded 

apices. Length of observed specimen, 67 ,u; width, 

251t. Valve surface covered with punctate transverse 

striae about 13 in 10,u. Axial and marginal striae 

are separated by four lyre-shaped hyaline horns extending from the hyaline central area. Axial area 

enclosing raphe isvery narrow. 

Remarks.-Navicula lyra occurs rarely in the assemblage, mostly as broken fragments of valves. 

Known geologic 1·ange.-Miocene to modern rna·· 

rine coastal waters, where it is common. 

Navicula pennata Schmidt 

Plate 7, figures 20, 21 

Navicula pennata Schmidt, 1876, Atlas der Diatomaceenkunde, 

pl. 48, fig. 41-43. 

Lohman, 1938, U.S. Geol. Survey Prof. Paper 189-C, p. 

84, pl. 22, fig. 16. 

Lohman, 1941, U.S. Geol. Survey Prof. Paper 196, part 3, 

p. 83-84, pl. 17, fig. 14. 

Hendey, 1964, An introductory account of smaller algae 
SYSTEMATIC DESCRIPTIONS 23 

of British coastal waters, Part V, Bacillariophyceae 

(Diatoms), p. 203, pl. 30, fig. 21. 

Descript1:on.-Valve linear-lanceolate, convex, 

with acutely rounded apices. Length 66f.L-78p.; width 

about 14f.L. Transverse striae coarse, widely spaced, 

about 5 in lOf.L. A quadrate hyaline central area is 

formed by the shortening of about two of the transverse striae at the center. Axial area narrow, terminating in small apical nodules. 

Re1narrks.-Na.vicula pennata is frequent in this 

assemblage. 

Known geologic range.-Miocene to modern seas, 

where it is a benthonic form. 

Genus DIPLONEIS Ehrenberg, 1845 

Diploneis crabro (Ehrenberg) Ehrenberg 

Plate 7, figures 22, 23, 

Pinnularia (Diploneis) cmbro Ehrenberg, 1844, Kgl. preuss. 

Akad. Wiss. Berlin, Ber., 1844, p. 85. 

Diploneis crabro (Ehrenberg) Ehrenberg, 1854, Mikrogeologie, pl. 19, fig. 29. 

Diploneis crabro Ehrenberg. Cleve, 1894, Kgl. svenska vetensk. 

akad. Handl., v. 26, no. 2, p. 100-102. 

Hustedt, 1937, Die Kieselalgen Deutschlands, 6sterreichs, 

und der Schweiz, pt. 2, p. 616-626, fig. 1028. 

Hendey, 1964, An introductory account of smaller algae 

of British coastal waters, Part V, Bacillariophyceae 

(Diatoms), p. 225, pl. 32, fig. 1, 3. 

DescripUon.-Valve panduriform with obtusely 

rounded apices. Length 31ft-45p.; width as great as 

l5p.. Central nodule longitudinally quadrate, extended to form siliceous horns on either side of the 

raphe. A ~single row of large puncta borders very 

narrow furrows. Transverse striae, 8 in lOp., radiate, 

consisting of double rows of fine puncta alternating 

with costae. 

Remarks.-This is a variable and complex species 

with a large number of named varieties. A related 

species, Diploneis prisca, has been previously reported from the Calvert Formation, but differs in 

having more tapered and less rounded lateral margins near the apices than does the D. C'rab1·o which 

occurs in this assemblage. 

Known geologic range.-A variety of D·1:ploneis 

c1·abro has been previously reported from the Calvert Formation of Miocene age. The species occurs 

in modern marine environments of high salinity. 

Subfamily PLEUROSIGMOIDEAE 

Genus PLEUROSIGMA Wm. Smith, 1852 

Pleurosigma affine var. marylandica Grunow 

Plate 7, figure 24 

Pleurosigma affine var. '11W1'ylanclica Grunow in Cleve and 

Grunow, 1880, Kgl. svenska vetensk. akad. Handl., v. 

17, no. 2, p. 51. 

Lohman, 1948, Maryland Dept. Geology, Mines, and Water Resources Bull. 2, p. 187. 

Plewrosigma normanii var. marylandica (Grunow) Cleve, 1894, 

Kgl. svenska vetensk. akad. Handl., v. 26, no. 2, p. 40. 

Boyer, 1904, Ma1·yland Geol. Survey, Miocene, p. 488. 

DescripUon.-Only the more robust central part 

of valves preserved in this assemblage. Width about 

30f.L. Axial area very narrow, central area small, 

rounded. Lateral parts of valve both ~convex between 

the raphe and the margins, covered with a grid of 

puncta, about 14 in lOf.L, arranged in both oblique 

and transverse striae, the oblique striae intersecting 

at an angle of about 60°. This pattern grades into 

a narrow band of exclusively transverse striae near 

the margin. 

Re1na1·ks.-The variety is rare in this assemblage. 

Known geologic range.-Previously reported only 

from the Calvert Formation of Miocene age. 

Family CYMBELLACEAE 

Amphora sp. 

Plate 7, figure 25 

Descr1:ption.-Valve lunate with gently curved 

dorsal side and slightly concav·e ventral side having 

a faint suggestion of a central swelling, tapering in 

width to rounded apices. Length of observed specimen, 56/-L; width, 9p.. Raphe paralleling the ventral 

side, flexed slightly toward that side near the center. 

Single rows of puncta paralleling the raphe on both 

sides. Puncta on dorsal side separated from dors,al 

striae by a thin hyaline band, extending nearly the 

length of the valve, but becoming obscure near the 

apices. Striae along dorsal side distinctly punctate, 

about 10 in lOf.L, curving around the dorsal edge of 

the valve as a mantle. 

Remarrks.-The figured specimen is the only com-

, plete valve observed in this assemblage. This appears to be a distinctive species of Amphm·a, which 

may have stratigraphic value, and it has not been 

previously described. Formal diagnosis, however, 

should await the study of additional, specimens. 

Known geologic 'tange.-This assemblage. 

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pt. 3: Am. Jour. Sci., v. 43, p. 321-332, pl. 3. 

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45, p. 313. 

--- 1844, Account of some new infusorial forms discovered 

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Piscataway, Maryland: Am. Jour. Sci., v. 46, p. 137-

141, pl. 1. 

--- 1845a, Ehrenberg's observations on the fossil Infusoria 

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Africa: Am. Jour. Sci., v. 48, p. 201-204. 
24 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION 

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Berggren, W. A., and Van Couvering, John, 1973, Late Neogene chronostratigraphy, biostratigraphy, biochronology, 

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v. 1, Leiden, Netherlands, E. J. Brill, p. 199-421. 

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Christian, Thomas, 1887, New diatomaceous deposits: The 

Microscope, v. 7, p. 65-68. 

Cooke, C. W., Gardner, J. A., and Woodring, W. P., 1943, 

Correlation of the Cenozoic formations of the Atlantic 

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Dall, W. H., and Harris, G. D., 1892, Correlation papers; 

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eastern Virginia and Maryland: Geol. Soc. America Bull., 

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Ehrenberg, C. G., 1843, Verbreitung und Einfluss des mikroskopischen Lebens in Siid- und N ord-Amerika: Kg!. 

preuss. Akad. Wiss. Berlin, Phys. Abh., 1841, p. 291-445, 

4 pis. 

--- 1844, Uber 2 neue Lager von Gebirgsmassen aus Infusorieri als Meeres-Absatz in Nord-Amerika und eine 

Vergleichung derselben mit den organischen Kreide-Gebilden in Europa und Afrika: Kgl. preuss. Akad. Wiss. 

Berlin, Ber., 1844, p. 57-97. 

--- 1845, Untersuchungen tiber die kleinsten Lebensformen 

in Quellenlande des Euph1·ats und Araxes, so wie tiber eine 

an neuen Formen sehr reiche marine Tripelbildung von 

den Bermuda-Inseln vor: Kgl. preuss. Akad. Wiss. Berlin, Ber., 1844, p. 253-275. 

--- 1854, Mikrogeologie: Leipzig, L. Voss, 374 p., 41 pl. 

Gernant, R. E., 1970, Paleoecology of the Choptank Formation 

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Gernant, R. E., Gibson,· T. G., and Whitmore, F. G., 1971, 

Environmental history of Maryland Miocene: Maryland 

Geol. Survey, Guidebook 3, p. 1-58. 

Gibson, T. G., 1967, Stratigraphy and paleoenvironment of the 

phosphatic Miocene strata of North Carolina: Geol. Soc. 

America Bull., v. 78, p. 631-650, 2 pis. 

Grunow, Albert, 1884, Die Diatomeen von Franz J osefs-Land: 

Kaisl.-Kgl. Akad. Wiss. Wien, Math.-Naturwiss. Kl., 

Denkschr., v. 48, p. 53-112, 5 pls. 

Hanna, G. D., 1932, The diatoms of Sharktooth Hill, Kern 

County, California: California Acad. Sci. Proc., 4th ser., 

v. 20, no. 6, p. 161-262, 18 pis. 

Hendey, N. I., 1937, The plankton diatoms of the southern 

seas: Discovery Repts., v. 16, p. 151-364, pls. 6-13. 

--- 1964, An introductory account of the smaller algae of 

British coastal waters, Part V, Bacillariophyceae (Diatoms) : London, Ministry of Agriculture, Fisheries and 

Food, Fishery Inv. Ser. 4, 317 p., 45 pls. 

Hustedt, Friedrich, 1927-30, 1931-59, 1961-66, Die Kieselalgen Deutschlands, 6sterreichs, und der Schweiz, Band 7 

of Rabenhorst, G. L., Kryptogamenflora * * *: Leipzig, 

Akademische Verlagsgesellschaft, pt. 1, 920 p., 542 figs., 

1927-30; pt. 2, 845 p., 637 figs., 1931-59; pt. 3, lieferungen 1-4, p. 1-816, figs. 1180-1788, 1962-66. 

Kanaya, Taro, 1959, Miocene diatom assemblages from the 

Onnagawa Formation and their distribution in the correlative formations in northeast Japan: Tohoku Univ. 

Sci. Repts., 2d ser. (Geology), v. 301

. p. 1-13,0, pls. 1-

11. 

Karsten, George, 1928, Bacillariophyta (Diatomeae), in Engler, Adolf, Die natiirlichen (Pflanzenfamilien; Band 2, 

Peridineae ( Dino:flagellatae) , Diatomeae ( Bacillariophyta), Myxomycetes: Leipzig, Wilhelm Engelmann, p. 

105-303, 329 figs. · 

Koizumi, Itaru, 1973, The Late Cenozoic diatoms of sites 183-

193, leg 19 Deep Sea Drilling Project, in Initial Reports 

of the Deep Sea Drilling Project, v. 19, Washington, D.C., 

U.S. Government Printing Office: p. 805-855, pls. 1-8. 

Lohman, K. E., 1938, Pliocene diatoms from the Kettleman 

Hills, California: U.S. Geol. Survey Prof. Paper 189-C, 

p. 81-94, pls. 20-23. 

--- 1942, Geology and biology of North Atlantic deep-sea 

cores between Newfoundland and Ireland, Part 3, Diatomaceae: U.S. Geol. Survey Prof. Paper 196, part 3, p. 

55-86, pls. 12-17. 

--- 1948, Middle Miocene diatoms from the 

W ell-Cr:etaceous and Tertiary subsurface 

Maryland Dept. Geology, Mines, and Water 

Bull. 2, p. 151-187, 331-333, pls. 5-11. 

Hammond 

geology: 

Resources, 

--- 1972, A procedure for the microscopical study of diatomaceous sediments: Nova Hedwigia, issue no. 39, p. 

267-283. 

Long, J. A., Fuge, D. P., and Smith, James, 1946, Diatoms 

of the Moreno Shale: Jour. Paleontology, v. 20, no. 2, p. 

89-118, pls. 13-18. 

Pantocsek, Josef, 1886, Beitrage zur Kenntnis der fossilen 

Bacillarien Ungarns, pt. 1: Berlin, W. Junk, p. 1-76, pls. 

1-30. 

Peragallo, H. and Peragallo, M., 1897-1908, Diatomees 

marines de France: Grez-sur-Loing, M. J. Tempere, 491 

p., pis. 1-137. 

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survey of the State of Virginia for the year 1840: Richmond, Virginia, 132 p. 

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Pacific, leg 18, in Initial Reports of the Deep Sea Drilling Project, v. 18: Washington, D.C., U.S. Government 

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Adolf, and Prantl, K. A. E., Die natiirlichen P:flanzenfamilien: Leipzig, pt. 1, Abt. 1b, p. 31-153, 282 figs. 

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[abs.] : Science, n. ser., v. 15, p. 906-907. 

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INDEX 

[Italic page numbers indicate descriptions and major references] 

Page 

A 

Achnanthaceae 2 2 

1::,~;:;:;;~ ~~: ::::::::::::::::::: 13 

14 

tenella _____________________ ~ ~ 14 

~llipticus --------------------- 6, 13; pl. 3 

mgens ----------------------- 6, 13; pl. 3 

moniliformis ------------ 14 ochotensis _ _ _ __ _ _ _ _ _ __ _ __ _ _ _ _ _ _ 13 

octonaritts ------------------·· 6, 14; pl. 3 

ocnlatus ------------------------- 13 

senari1tS 

ten ell us 

---------- 15 

--------------- 6, 14; pl. 3 

tsugarnensis 13 

!:~!::~~~:::~:a~~~~~~~~~~~~~~~~------- 14 

Actinoptychus ---~~~~~~~~~~~~~~~~~~~~ ~! marylandicus ------------- 6, 14, 12; pl. 4 

:e~~a:~~~t~ts --_ ~------~ ~--~ ----~--~---_-_-_-_-__ 6: -~5; p\! 

virginicus -------------------- 6, 15; pl. 4 

vulgaris australis ---------------- 15 

virginiae _________ _ 15 

virginiea 15 

actinoptychus, Ma;t-o-;;;,~i-;:-~~~~~=~=---- 19 

Stephanogonia _______ -;;,-;9; pl. 6 

affine marylandica, Pleurosigma ___ 6, 23; pl. 7 

Age of formations ------·---~---------- 3 

americana, Psettdopyxilla --------- 6, 19; pl. 6 

Pyxilltt -------------------------- 19 

Rhizosolenia ------------- 19 

~~;~:~~~:~! ~~-- ~~~ ~~~~ ~~~~~~~---_ -~·- ~7; p\ ~ amphiccros, Fragilaria ---------------- 21 

Amp~~~;~h:~ei:--~~~~=~============ ~: !~~ ~!: ~ angulosa, Grarnmatoph(lra _____ 6, :!1, 22; pl. 7 

an~nstata, Rhaphoncis ------------ 6, :!0; pl. 7 

aptculatns, Coscinodiscus _________ 6, 10; pl. 2 

Ara.phidineac ----------------------- 20 

arctica inermis, Stephanopyxis 

turris -------------------- 9 

aspcrulus, Lira-discus _____________ 6, 16; pl. 5 

Auliscineae _______ -~ _ __ __ _ _ _ __ __ __ _ _ _ _ 16 

aurita obtnsa, Bidclnlphia _________ 6, 16; pl. 5 

australis, Actinoptychus vulgaris 15 

B 

Biddttlphia -------------------- 16 

aurita obtnsa ----------------- 6, 16; pl. 5 

condccora ------------------------ 18 

intcrpunctaturn ------------------- 18 

ropcriana ----------- ____ 16 

scrnicircularis ---------------- 6, 16; pl. 5 

tridcns --------------------------- 17 

tuomcyi ---------------------- 6, 17; pl. 5 

Biddulphiaceae ----------- 16 

::~~~!~~:~~::e -=~ ===~==~= =~~==~~~~~~~~~ Biddulphioideae ______________________ _ 

Biraphidineae 

hrigh twclli, Enodia 

16 

16 

16 

16 

Page 

c 

Calvert Cliffs locality --------------- 1, 2 

Calvert diatom assemblage ----------- 6 

Calvert Formation ------------- 1, 2, 3, 5, 6, 7 

Fairhaven Diatomaceous Earth Member __________________ _ 

Plum Point Marl Member 2, 

cellnlosa, Coscinodiseus --------

perforatus -------- 6, 11, 12; pl. 2 Centrales _ __ ___ ___ ____ __ ___ __ __ __ _ __ __ 8 

Chancellors Point locality -------------- 2 

Chesapeake Group ----------------- 1, 2, 3, 4 

composition ----------------- ____ 2 

Choptank diatom assemblage _________ 6, 7 

Choptank Formation ___________ 1, 2, 3, 4, 5, 7 

Chrysophyta -------------------------- 8 

Cocconeis ----------------------------- ::!2 

sp --------------------------- 6, 2::!; pl. 7 

Cocconeoideae ------------------------ 22 

complcxa, M closira. __________________ _ 

Paralia ----------------------- 6, 8; pl. 1 

condccora, Biddulphia ----------- 18 

condccorum, Tric~ratium 6, 18; pl. 5 

corona, Stcpha.nopy:tis ------------- 6, 9; pl. 1 

Systchania ----------------------- 9 

coronata, Gaillonclla ------------------

Melosira sulcata. ------------------ 9 

Melosira (Paralia) s11lcata ________ 9 

Paralia sulcata ---------------- 6, 9; 1)1. 1 

Coscinodiscaceae _ _ _ ___ _ __ ___ __ _ _ _ __ __ _ 11 

Coscinodi8coideae ____________________ _ 10 

Cvscinodiscus _________________________ 10, 13 

apicnlatns ------------------- 6, 10; pl. 2 

cnrvatulus -------------------- 6, 10; pl. 2 

dcgans --------------------------- 1B 

inc/usus --------------------- 6, 10; pl. 2 lineatus __________________________ _ 

nwrginatus ------------------ 6, 11; pl. 2 

oculus-iridis ------------------ 6, 11; pl. 2 

pavillardi ------------------------ 12 

pcrfvratus ------------- 6, 10, 11, 18; pl. 2 

cellulosa -------------- 6, 11; 12; pl. 2 

pavillardi ---------------- 6, 1:!; pl. 2 

radiatus --------------------- 6, 1:!; pl. 2 

rothii ------------------------- 6, 1:.!; pl. 3 

subtilis _____ .. _______ _____ __________ 12 

vctnstissimus ----------------- 6, 1:!; pl. 3 

coscinodiscus, Craspcdvdiscus _____ 6, 13; pl. 3 

crabro, Di}Jloncis ----------------- 6, ,.!;?; pl. 7 

Pinnularia (Diploneis) ___________ 23 

Craspcdoclzscus ------------------------ 13 

coscinodiscus _________________ 6, 13; pl. 3 

microdiscus ---------------------- 13 

Creswcll1a turris ---------------------- 10 

cruciata, Dictyopyxis __________________ 9 

Pyxidicula -------------------- 6, 9; pl. 1 

curvatnl-ns, Coscinodiscus _________ 6, 10; pl. 2 

Cymbellaceae _______________________ _ 

D 

Depositional environment ______________ 1, 7 

diamantclla, Rhaphoneis __________ 6, 20; pl. 6 

Diatom assemblage -------------------- 6 

stratigraphic significance _________ 6 

Diatoma. marinnm --------------------

Diatomophyceae _ ---------- ___________ _ 

Dictyopyxis cruciata ------------------

Page 

22 

Diploncis ----------------------------- 23 

crabro ----------------------- 6, :!3; Pl. 7 p r is ca. ___________________________ _ 

(Diploncis) crabro, Pinnularia ______ _ 

23 

23 

Discineae ___ _ __ ___ ___ ___ ___ __ ___ ___ ___ 8 

Dossctia ------------------------------ 19 

hyalina ---------------------- 6, 19; pl. 6 tempcrei __ __ _ __ __ ___ _ __ _ _ _ __ _ __ __ 19 

E 

eh rcnbcrgii, Actinocyclus 

tcnclla, Actinocyc/us 

14 

14 

clcgans, Coscinodiscus 13 

ellipticus, A.ctinocyclus 6, 13; pl. 3 

Eucampia -------------------------- 17' 

l'irginic<t --------------------- 6, 17; pl. 5 zood iacns _______________________ _ 

Eucampiaceae _______________________ _ 

Ezwdia urightwclli ------------------~- Eupodiscaceae ________________________ _ 

Eupodiscineae 

Eupodiscoideae ______________________ _ 

Eupodiscus inconspiC1WS --------------

Eupodiscus tcncllus -------------------

F 

Fairhaven Diatomaceous Earth Member 

Calvert Formation _______ _ 

Frnailaria a mph iccros -----------------

Fragilariaceae 

Fragilarioideae ______________________ _ 

G 

Gaillvnclla coronata 

sulcata ---------------------------

{lcmmifaa, Rhap/zoncis 

Gonioth<ciunz ___________________ _ 

17 

17 

16 

16 

16 

16 

16 

14 

2 

21 

20 

20 

9 

20 

17 

rogcrsii ____ ------------------ 6, 17; pl. 5 

Grammatoplzora ---------------------- :!1 

anrndo.~a ------------------ 6, :!1, 22; pl. 7 

marina ----------------- 6, ;!,:,>;pl. 7 

H 

Hammond well Hemiaulaceae ________________________ _ 

Hemiauliineae ______________________ _ 

5 

18 

18 

H on iazdus _ _ __ ___ _ ___ ___ __ _ ____ _ ____ _ 18 

polynzon;hus 1•irginica ------------ 6, 18 

hnuz:'d!li, Na!'icula --------------- 6, 2:!; pl. 7 Hcterales _ ___ _ __ _ _ _ _ _ _ _ _ ___ _ _ ___ _____ _ 18 

Heterineae __________________________ _ 18 

II ctcrostcphan 12 ia roth ii _______________ _ 

lzyalina, Dossctia ------------------

Hyalodiscns ---------------------

lacvis 

6, 19; pl. 6 

14 

6, 14; pl. 4 

immzwis, Rhaphoncis ------------- 6, 21; pl. 7 

incfusus, Coscinodiscus ___________ 6, 10; pl. 2 

25 
26 

Page 

inconspicua, Rattra.yel/a ---------- 6, 16; pl. 5 

inconspicuus, Eupodiscus ------------- 16 

incrmis, Stcphanopyxis tnrris arctica___ 9 

ingcns, Actinocyclus -------------- 6, 13; pl. 3 

intcrpunctatum, Biddulphia ___________ 18 

Tricc1·atium ---------------------- 18 Introduction _________________________ _ 

L 

lacvis, Hya/odiscus ---------------- 6, 14; pl. 4 

lanecttula, Rhaphoncis --------- 6, 20, 21; pl. 7 

lincata, Pcristcphania ----------------- 9 

Stcphanopyxis ----------------- 6, 9; pl. 1 

lincatus, Coscinodiscus ---------------- 9 

Stcphanodiscus __________________ _ 

Liradiscaceae ------------------------- 16 

Liradiscoideae _ __ _ __ ___ ____ ___ __ __ _ _ __ 16 

Liradiscus --------------------------- 16 

aspcrulus -------------------- 6, 16; pl. 5 

ovalis ------------------------ 6, 16; pl. 5 

longissima, Thalassiothrix _________ 6, :31; pl. 7 

lyra, Navicula -------------------- 6, 22; pl. 7 

M 

marginatus, Coscinodiscus ________ 6, 11; pl. 2 

marina, Grammatophora -----··---- 6, 22; pl. 7 

mariwon, Diatoma -------------------- 22 

marylandica, Plcurosigma a_ljinc ___ 6, o!J; pl. 7 

Pln<rosigma normanii ------------ 23 

marylandicus, Actinoptychus ___ 6, 14, 15; pl. 4 

Raph idodiscus ___ _ __ _ __ ___ __ __ _ __ _ 5 

Mastogonia actinoptyclms ------------ 19 

Mdosira ------------------------------ 8 

cumplc.ra ------------------------ 8 su l c a ta __________________________ _ 

coronata ----------------------

wcstii ------------------------- 6, 8; pl. 1 

(Paralia) sulcata coronata ________ 9 

Melosiroideae ------------------------- 8 

Method used in study ----------------- 6 

microdiscHs, Craspcdodiscus ------------ 13 

microspinosa, Xanthiopyxis _______ 6, 18; pl. 6 

moniliformis, Actinocyclus ------------ 14 

Monoraphidineae --------------------- 22 

N 

Navicula ----------------------------- 22 

hcnncdyi -------------------- 6, £?2; pl. 7 

lyra ------------------------- 6, 22; pl. 7 

pennata ---------------------- 6, 22; pl. 7 

Naviculaceae ------------------------- 22 

N a viculoideae 

nitzschioides, Thalassionema ----------- 21 

obtusa, Thalassiothrix ------------ 21 

normanii mary/andica, P/curosigma 23 

0 

obtusa, Biddulphia aurita ________ 6, 16; pl. 5 

Odontdla ------------------------- 16 

'l'halassioncma ---------------- 6, 21; pl. 7 

Thalassiothrix nitzschioides _______ 21 

ol,tusum, Tric•Tatium ---------------- 16 

ochotcnsis, Actinocycfus --------------- 13 

octonarius, Actinocyc/us ---------- 6, 14; pl. 3 

oculatus, Actinocyclus ----------------- 13 

oculus-iridis, Coscinodiscus ________ 6, 11; pl. 2 

Odontclla obtusa --------------------- 16 

ovalis, Liradiscus 6, 16; pl. 5 

p 

Paleoecologic interpretation 7 

Paralia -------------------------------- 8 

complcxa --------------------- 6, 8; pl. 1 

sulcata 

coronata 

6, 8; pl. 1 

6, 9; pl. 1 

INDEX 

Page 

(Paralia) sulcata coronata, Melosira __ 9 

pavillardi, CoscinodiscHs -------------- 12 

pavillardi, Coscinodiscus pcrforatus_ 6, 1:2; pl. 2 

Pennales ----------------------------- :20 

pennata, Navicula ______ ---------- 6, 22; pl. 7 

pcrforatus, Coscinodiscus ___ 6, 10, 11, 18; pl. 2 

cellulosa, Coscinodiscus ____ 6, 11, 12; pl. 2 

pavillardi, Coscinodiscus ______ 6, 12; pl. 2 

Pcriptcra ------------------------------ 19 

tetracladia ------------------- 6, 19; pl. 5 

Pcristephania lineata ----------------- 9 

Pinnularia (Diploncis) crabro -------- 23 

Plcurosigma --------------------------- 23 

affine marylandica ------------ 6, 2J; pl. 7 

normanii marylanclica ------------ 23 

Pleurosigmoideae --------------------- 23 

Plum Point Marl Member, Calvert 

Formation ---------------

polyacantha, Stcplwnogonia __________ _ 

po/ymorplms virginica, Hemiaulus ____ _ 

Previous work ____________________ ---

prisca, Diploneis _____________________ _ 

Protista _____________________________ _ 

Protoctista __________________________ _ 

Pseudopyxilla ________________________ _ 

2, 3 

19 

6, 18 

4 

23 

19 

americana -------------------- 6, 19; pl. 6 

Pyxidicztla ---------------------------- 9 

cruciata ------------------------ 6, 9; pl. 1 

Py:n·lla. americana --------------------- 19 

Pyxillaceae ____ ------------ ----------- 19 

R 

radiatus, Coscinodiscus ------------ 6, 12; pl. 2 

Raphidod1scus marylandicus ----------- 5 

Rattraycl/a --------------------------- 16 

inconspicua ------------------ 6, 16; pl. 5 

Rhaphoncis -------------------------- 20 

amphiccros ------------------- 6, 20; pl. 6 

angustab -------------------- 6, 20; pl. 7 

diamantclla ------------------ 6, 20; pl. 6 gem mifera _ _ ___ __ _ _ __ __ _ __ _ ___ ___ 20 

immunis --------------------- 6, 21; pl. 7 

lancettula ---------------- 6, 20, 21; pl. 7 

Rhizosolenia americana ---------------- 19 

rogcrsii, Goniothecium ------------ 6, 17; pl. 5 

ropc.riana, Biddulphia ----------------- 16 

rothii, Coscinodiscus -------------- 6, 12; pl. 3 

Hetcrostephania ------------------ 12 

s 

St. Marys Formation ---------------- 1, 2, 3 

Selected references ------------------- 23 

scmicirculare, TriCt'ratiurn ------------ 16 

scmicircularis, Bidfl!tlphia ________ 6, 16; pl. 5 

scnarius, Actinocyclus ________ _________ 15 

Actinoptychus ---------------- 6, 15; pl. 4 

Sen-avalian Stage of middle Miocene age 4 

Stcplwnodiscus lincatns -------------- 9 

Strphanogonia ------------------------ 19 

act.inoptyc/ws ---------------- 6, 19; pl. 6 

po/yacantlw. ----------------------- 19 Stepha nopy;ris _______________________ _ 

corona ------------------------ 6, 9; pl. 1 

lincata ------------------------- 6, 9; pl. 1 

turris ------------------------ 6, 10; pl. 2 

arctica incrrnis --------------- 9 

Stratford Cliffs locality ---------------

Stratigraphic significance of diatom 

assemblage ______________ _ 

Stratigraphic zones and formations __ _ 

Substratum ___________________________ _ 

subtilis, Coscinodiscus -----------------

2 

12 

Page 

snlcata, Gai/loncfla -------------------- 8 

Melosira -------------------------- 8 

Paralia ----------------------- 6, 8; pl. 1 

coronnta, Melosira ---------------- 9 

Melosira (Paralia.) ------------ 9 

Paralia ------------------- 6, 9; pl. 1 

Synedra thalassiothrix ----------------- 21 

Systchania corona. -------------------- 9 

Systematic descriptions ---------------- 7 

T 

Tabellarioideae 21 

t~ mp,rci, Dossctia -------------------- 19 

tcne/la, Actinocyclus chrcnbergii _______ 14 

tcncllus, Actinocyclus ------------- 6, 14; pl. 3 

EHpodiscus ----------------------- 14 Terpsinoaceae ___ _ ___ __ _ __ _ _____ __ _ __ _ 17 

Tcrpsinoc __ __ __ __ _____ __ ___ __ _____ __ _ 17 

amrricnnn ----------------··-- 6, 17; pl. 5 

tctracladia, Pcriptcra ------------- 6, 19; pl. 5 

Tctragrarnma americana -------------- 17 

'l'lwlassioncma ------------------------ 21 

nitzschioides ---------------------- 21 

obtusa ----------------------- 6, 21; pl. 7 

Tlwlassiothrix ------------------------- 21 

longissima ------------------- 6, 21; pl. 7 

nitzschioides obtusa --------------- 21 

thalassiothrix --------------------- 21 

thalassiothrix, Synedra --------------- 21 

Thalassiothrix -------------------- 21 

Tricerataceae ------------------------- 18 

Triceratiineae ------------------------ 18 

Triccratium -------------------------- 18 

condccorum ------------------ 6, 18; pl. 5 

interpuncta.tum ------------------- 18 

obtusum -------------------------- 16 

scmicircularc --------------------- 16 

triclens, Bicldulphia ------------------- 17 

tsugaruensis, Acthzocyclus ------------- 13 

tuomcyi, Biddulphia, _______________ 6, 17; pl. 5 

Zygoceros ------------------------ 17 

turris, Crcsu-ellia --------------------- 10 

Stcphanopyxis ---------------- 6, 10; pl. 2 

a1·ctica inerm is, Stephanopyxis 9 

u 

Unconformities ----------------------- 2 

undulatu.~. Actinoptychus -------------- 15 

v 

vetustissimus, Coscinodiscns _______ 6, 12; pl. 3 

virginia.c, Actinoptyclms vulgaris ______ 15 

virginica 1Actinoptychus vulgaris ______ 15 

Euca.m]Jia -------------------- 6, 17; pl. 5 

Hcmiaulus polyrnorphus ------------ 6, 18 

virginicus, Actinoptychus _________ 6, 15; pl. 4 

vulgaris australis, Actinoptychus ______ 15 

virginia.c, Actinoptychus ----------- 15 

virginica.:sActinoptychus________ ____ 15 

w 

wcstii, Melosira ------------------- 6, 8; pl. 1 

Windmill Point locality ____ ----------- 2 

X 

Xanthiopyxaceae _____________________ _ 

X a nth iopyxis ________________________ _ 18 

6, 18 

rnicrospinosa ------------------ 6, 18; pl. 6 

spp ------------------------------ 6,19 

z 

zoodiacns, Eucampia ------------------

Zygocrros Tuomeyi -------------------

17 

17 

*U.S.GOVERNMENT PRINTING OFFICE: 1!:176 0-211-317/38 
PLATES 1-7 

Contact photographs of the plates in this report are available, at 

cost, from U.S. Geological Survey Library, Federal Center, 

Denver, Colorado 80225 
PLATE 1 

FIGURE 1, 2. Melosira westii W. Smith, X 960, (p. 8). 

1. USGS diatom cat. No. 3902-11. Diameter, 30,u. 

2. USGS diatom cat. No. 3901-21. Diameter, 27,u. 

3, 4. Paralia cornplexa (Lohman) Andrews, X 960, (p. 8). 

3. USGS diatom cat. No. 3903-9. Diameter, 52,u. 

4. USGS diatom cat. No. 3903-8. Diameter, 53,u. 

5, 6. Pat·alia sulcata (Ehrenberg) Cleve, X 960, (p. 8). 

5. USGS diatom cat. No. 3900-1. Diameter, 38,u. 

6. USGS diatom cat. No. 3771-1. Diameter, 36,u. 

7, 8. Pwralia sulcata var. coronata (Ehrenberg) Andrews, X 960, (p. 9). 

7. USGS diatom cat. No. 3900-2. Diameter, 33,u. 

8. USGS diatom cat. No. 3901-2. Diameter, 27,u. 

9, 10. Pyxidicula cruciata Ehrenberg, X 960, (p. 9). 

9. USGS diatom cat. No. 3772-47. Diameter, 44,u. 

10. USGS diatom cat. No. 3772-11. Diameter, 31,u. 

11, 12. Stephanopyxis corona (Ehrenberg) Grunow, X 960, (p. 9). 

11. USGS diatom cat. No. 3901-16. Diameter, 74,u. 

12. USGS diatom cat. No. 3901-26. Diameter, 60,u. 

13, 14. Stephanopyxis lineata (Ehrenberg) Forti, X 960, (p. 9). 

13. USGS diatom cat. No. 3900-28. Diameter, 64,u. 

14. USGS diatom cat. No. 3772-46. Diameter, 52,u. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 PLATE 1 

MELOSIRA, PARALIA, PYXIDICULA, AND STEPHANOPYXIS 
PLATE 2 

FIGURE 1, 2. Stephanopyxis turris (Greville and Arnott) Ralfs, X 960, (p. 10). 

1. USGS diatom cat. No. 3903-12. Diameter, 301'. 

2. USGS diatom cat. No. 3771- 12. Diameter, 251'. 

3. Coscinocliscus apiculatus Ehrenberg, X 475, (p. 10). 

USGS diatom cat. No. 3900-20. Diameter, 117.u. 

4. Coscinocliscus curvatulus Grunow, X 960, (p. 10). 

USGS diatom cat. No. 3904-2. Diameter 481'. 

5. Coscinocliscus inclusus Rattray, X 960, (p. 10). 

USGS diatom cat. No. 3771-13. Diameter, 381'. 

6, 7. Coscinocliscus marginatus Ehrenberg, X 475, (p. 11). 

6. USGS diatom cat. No. 3772-56. Diameter, 901'. 

7. USGS diatom cat. No. 3771- 29. Diameter, 91.u. 

8. Coscinocliscns oculus-it'iclis Ehrenberg, X 228, (p. 11). 

USGS diatom cat. No. 3772-39. Diameter, 1961'. 

9. Coscinocliscus perfomtus Ehrenberg, X 475, (p. 11). 

USGS diatom cat. No. 3900-2G. Diameter, 1021'. 

10. Coscinocliscw; perforatus var. cellulosa Grunow, X 475, (p. 11). 

USGS diatom cat. No. 3903-3. Diameter, 811'. 

11, 12. Coscinocliscns perforatus var. pavillat·cli (Forti) Hustedt, X 475, (p. 12). 

11. USGS diatom cat. No. 3772-27. Diameter, 1111'. 

12. USGS diatom cat. No. 3772-23. Diameter, 1181'. 

13. Coscinocliscus mcliatus Ehrenberg, X 475, (p. 12). 

USGS diatom cat. No. 3901- 17. Diameter, 1001'. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 PLATE 2 

2 

1 

4 

STEPHANOPYXIS AND COSCINODISCUS 
FIGURE 

PLATE 3 

1, 2. Coscinodiscus rothii (Ehrenberg) Grunow, X 4 75, (p. 12). 

1. USGS diatom cat. No. 3771-33. Diameter, 58,u. 

2. USGS diatom cat. No. 3772-28. Diameter, 67,u. 

3. Coscinodiscus vetustissimus Pantocsek, X 960, (p. 12). 

USGS diatom cat. No. 3901-44. Diameter, 58,u. 

4. Craspedocliscus coscinocliscus Ehrenberg, X 475, (p. 13) . 

USGS diatom cat. No. 3772-13. Diameter, 74,u. 

5, 6. Actinocyclus ellipticus Grunow, X 960, (p. 13). 

5. USGS diatom cat. No. 3900-39. Length, 75,u. 

6. USGS diatom cat. No. 3901-33. Length, 79,u. 

7. Actinocyclus octonarius Ehrenberg, X 475, (p. 14). 

USGS diatom cat. No. 3771-26. Diameter, 98,u. 

8, 9. Actinocyclus tenellus (Brebisson) Andrews, X 960, (p. 14). 

8. USGS diatom cat. No. 3901-18. Diameter, 70,u. 

9. USGS diatom cat. No. 3901-46. Diameter, 561'· 

10. Actinocyclus ingens Rattray, X 960, (p. 13). 

USGS diatom cat. No. 3902-4. Diameter, 53,u. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 PLATE 3 

COSCINODISCUS, CRASPEDODISCUS, AND ACTINOCYCLUS 
FIGURE 

PLATE 4 

1, 2. Hyalocliscus laevis Ehrenberg, X 960, (p. 14). 

1. USGS diatom cat. No. 3903- 6. Diameter, 53,u. 

2. USGS diatom cat. No. 3904-5. Diameter, 40,u. 

3- 6. Actinoptychus marylandicus Andrews, X 960, (p. 14). 

3, 4. Holotype, USGS diatom cat. No. 3904-12. Diameter, 54,u. 

5, 6. Paratype, USGS diatom cat. No. 3901-28. Diameter, 591'. 

7, 8. Actinoptychus senm·ius (Ehrenberg) Ehrenberg, X 960, (p. 15). 

USGS diatom cat. No. 3902-15. Diameter, 511'. 

9-12. Actinoptyclms vi1·ginicus (Grunow) Andrews, X 960, (p. 15). 

9, 10. USGS diatom cat. No. 3901-27. Diameter, 65/L. 

11, 12. USGS diatom cat. No. 3902-1. Diameter, 35,u. 
GEOLOGICAL SURVEY 

PROFESSIONAL PAPER 910 PLATE 4 

HYALODISCUS AND ACTINOPTYCHUS 
FIGURE 

PLATE 5 

1, 2. Rattrayella inconspicua (Rattray) Hanna, X 475, (p. 16). 

1. USGS diatom cat. No. 3772-1. Diameter, 741-'. 

2. USGS diatom cat. No. 3772- 43. Diameter, 701-'. 

3-5. Limdiscus asperulus Andrews, X 960, (p. 16). 

3. Holotype, USGS diatom cat. No. 3903-7. Length, 401-'. 

4. Paratype, USGS diatom cat. No. 3904- 6. Length, 431-'. 

5. Paratype, USGS diatom cat. No. 3902-4. Length, 23!-'. 

6, 7. Limdisc-us ovalis Greville, X 960, (p. 16). 

6. USGS diatom cat. No. 3904- 7. Length, 221-'. 

7. USGS diatom cat. No. 3903-8. Length, 301-'. 

8. Biddulphia aw·ita var. obtusa (Ki.itzing) Hustedt, x 475, (p. 16). 

USGS diatom cat. No. 3772-34. Length, 471-'. 

9, 10. Biddulphia semici1·ctdaris (Brightwell) Boyer, X 960, (p. 16) . 

9. USGS diatom cat. No. 3901-20. Length, 45!-'. 

10. USGS diatom cat. No. 3771- 5. Length, 701-'. 

11. Biddulphia tuomeyi (Bailey) Roper, X 960, (p. 17). 

USGS diatom cat. No. 3901-13. Length, 571-'. 

12, 13. Goniothecium 1·ogm·sii Ehrenberg, X 475, (p. 17). 

12. USGS diatom cat. No. 3771-11. Length, 801-'. 

13. USGS diatom cat. No. 3771-17. Length, 68,u. 

14, 15. Eucam11ia vi1·ginica Grunow, X 960, (p. 17). 

14. USGS diatom cat. No. 3900- 40. Length, 621-'. 

15. USGS diatom cat. No. 3772-54. Length, 761-'. 

16, 17. TeTpsino e ame1·icana (Bailey) Ralfs, X 960, (p. 17). 

16. USGS diatom cat. No. 3772-16. Length, 711-'. 

17. USGS diatom cat. No. 3772-32. Length, 611-'. 

18, 19. Trice1·atimn condecoTum Ehrenberg, X 960, (p. 18). 

18. USGS diatom cat. No. 3771-23. Side, 441'. 

19. USGS diatom cat. No. 3900-40. Side, 461-'. 

20, 21. Periptera tetmcladia Ehrenberg, X 960, (p. 19) . 

20. USGS diatom cat. No. 3903-2. Length, 321-'. 

21. USGS diatom cat. No. 3905-3. Length, 281'. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 PLATE 5 

9 

11 

13 

20 21 

RATTRA YELLA, LIRADISCUS, BIDDULPHIA, GONIOTHECIUM, 

EUCAMPIA, TERPSINOE, TRICERATIUM, AND PERIPTERA 

6 

15 
PLATE 6 

FIGURE 1-3. Xanthiopy xis microsp'inosa Andrews, X 960, (p. 18). 

1. Holotype, USGS diatom cat. No. 3901-43. Length, 521-'. 

2. Paratype, USGS diatom cat. No. 3772-37. Length, 32,u. 

3. Paratype, USGS diatom cat. No. 3900-46. Length, 321-'. 

4-7. Dossetia hyalina Andrews, X 960, (p. 19). 

4. Holotype, USGS diatom cat. No. 3901-38. Length, 451-'. 

5. Paratype, USGS diatom cat. No. 3901-19. Length, 371-'. 

6. Paratype, USGS diatom cat. No. 3901-29. Length, 40,u. 

7. Paratype, USGS diatom cat. No. 3904-3. Length, 421-'. 

8. StC?Jhanogonia actinoptychus (Ehrenberg) Grunow, X 960, (p. 19). 

USGS diatom cat. No. 3771-28. Length, 62.u. 

9-12. Psendopyxilla ame1·icana (Ehrenberg) Forti, X 960, (p. 19). 

9. USGS diatom cat. No. 3902-5. Length, 73,u. 

10. USGS diatom cat. No. 3903-11. Length, 41,u. 

11. USGS diatom cat. No. 3902-9. Length, 35/-'. 

12. USGS diatom cat. No. 3904-10. Length, 551'. 

13, 14. Rhaphoneis amphice1·os Ehrenberg, X 960, (p. 20). 

13. USGS diatom cat. No. 3772- 22. Length, 711'. 

14. US GS diatom cat. No. 3772- 2. Length, 45.u. 

15-18. Rhaphoneis dianw.11 tella Andrews, n. sp. X 960, (p. 20). 

15. Pa1·atype, USGS datom cat. No. 3771-9. Length, 40,u. 

16. Paratype, USGS diatom cat. No. 3772-52. Length, 501'. 

17. Holotype, USGS diatom cat. No. 3772- 29. Length, 62,u. 

18. Pa1·atype, USGS diatom cat. No. 3900- 14. Length, 70,u. 
GEOLOGICAL SURVEY 

9 

10 

2 3 

6 

XANTHIOPYXIS, DOSSETIA, STEPHANOGONIA, 

PSEUDOPYXILLA, AND RHAPHONEIS 

PROFESSIONAL PAPER 910 PLATE 6 

5 
PLATE 7 

FIGURE 1, 2. Rhaphoneis angustata Pantocsek, X 960, (p. 20). 

1. USGS diatom cat. No. 3772-25. Length, 53,u. 

2. USGS diatom cat. No. 3901-51. Length, 58,u. 

3. Rhaphoneis immunis Lohman, X 960, (p. 21) . 

USGS diatom cat. No. 3772-50. Length, 45,u. 

4, 5. Rhaphoneis lancettula Grunow, X 960, (p. 21). 

4. USGS diatom cat. No. 3771- 25. Length, 68,u. 

5. USGS diatom cat. No. 3771-31. Length, 70,u. 

6-8. Thalassionema obtusa (Grunow) Andrews, X 960, (p. 21). 

6. USGS diatom cat. No. 3771-10. Length, 36,u. 

7. USGS diatom cat. No. 3771-2. Length, 35,u. 

8. USGS diatom cat. No. 3772- 55. Length, 32,u. 

9, 10. Thalassiothrix longissirna Cleve and Grunow, X 960, (p. 21). 

9. USGS diatom cat. No. 3904-13. Length of fragment, 76,u. 

10. USGS diatom cat. No. 3905-4. Length of fragment, 75,u. 

11-13. Gmmmatophom angulosa Ehrenberg, X 960, (p. 21). 

11. USGS diatom cat. No. 3900- 15. Length, 51,u. 

12. USGS diatom cat. No. 3900-45. Length, 38,u. 

13. USGS diatom cat. No. 3901-41. Length, 27,u. 

14, 15. Grarnmatophora rnaTina (Lyngbye) Kiitzing, X 960, (p. 22). 

14. USGS diatom cat. No. 3903- 5. Length, 44,u. 

15. USGS diatom cat. No. 3901-39. Length, 38,u. 

16. Cocconeis sp., X 960, (p. 22). 

USGS diatom cat. No. 3902-16. Length, 41,u. 

17, 18. Navicula henned-y·i W. Smith, X 960, (p. 22). 

17. USGS diatom cat. No. 3900-44. Length, 58,u. 

18. USGS diatom cat. No. 3900-32. Length, 48,u. 

19. Navicula lyra Ehrenberg, X 960, (p. 22). 

USGS diatom cat. No. 3772-61. Length, 67,u. 

20, 21. Navicula pennata Schmidt, X 960, (p. 22). 

20. USGS diatom cat. No. 3772-57. Length, 7811. 

21. USGS diatom cat. No. 3771-32. Length, 79/1. 

22, 23. Diploneis cmbTo (Ehrenberg) Ehrenberg, X 960, (p. 23). 

22. USGS diatom cat. No. 3900-41. Length, 51,u. 

23. USGS diatom cat. No. 3903-4. Length, 50,u. 

24. Pleur-osigma.affine var. mat·ylandica Grunow, X 960, (p. 23). 

USGS diatom cat. No. 3902-6. Length of fragment, 60,u. 

25. AmphO?·a sp., X 960, (p. 23). 

USGS diatom cat. No. 3901-49. Length, 5611 . 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 PLATE 7 

6 7 

3 

2 

5 

13 

15 

RHAPHONEIS, THALASSIONEMA, THALASSIOTHRIX, GRAMMATOPHORA, 

COCCONEIS, NAVICULA, DIPLONEIS, PLEUROSIGMA, AND AMPHORA