Chesapecten, a New Genus of Pectinidae 

(Mollusca: Bivalvia) From the Miocene 

and Pliocene of Eastern North America 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 861 

Chesapecten) a New Genus of Pectinidae 

(Mollusca: Bivalvia) From the Miocene 

and Pliocene of Eastern North America 

By LAUCK W. WARD and BLAKE W. BLACKWELDER 

GEOLOGIC.AL SURVEY PROFESSIONAL PAPER 861 

A study of a stratigraphically important 

group of Pectinidae with recognition of 

the earliest described and figured 

Anzerican fossil 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1975 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Ward, Lauck W 

Chesapecten, a new genus of Pectinidae (Mollusca, 

Bivalvia) from the Miocene and Pliocene of eastern 

North America. 

(Geological Survey professional paper ; 861) 

Includes bibliography and index. 

Supt. of Docs. no.: I 19.16:861 

1. Chesapecten. 2. Paleontology-Tertiary. 

3. Paleontology-North America. I. Blackwelder, 

Blake W., joint author. II. Title III. Series: 

United States. Geological Survey. Professional 

paper : 861. 

QE812.P4W37 564'.11 74-26694 

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

Washington, D.C. 20402- Price $1.45 (paper cover) 

Stock Number 2401-02574 
CONTENTS 

Page 

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

Introduction -------------------------------------------------.----------------------------- 1 

Acknowledgments ______________________________ -------- __________________________ ------ _ _ _ 3 

Summary of past work __________________________________ -.- ________ -------------- _ ___ _ __ _ _ _ 3 

Stratigraphic ·succession of Chesapecten ___ _ _ _ ___ _ ___ _ _ __ _ _ _ __ _ _ _ __ _ _ __ _ _ _ __ _ _ _ __ _ ___ _ ___ _ _ _ 4 

R·emarks ________________________________________ ---.- ______________ ---------- _ __ _ __ _ _ __ _ _ _ 6 

Systematic descriptions ________________________________________________________ -------- _ _ _ 7 

Genus Chesapecten Ward and Blackwelder, new genus ------------------------------------ 7 

Chesapecten coccymelus (Da.ll) ---------------------------------------------------- 8 

C. nefrens Ward and Blackwelder n. sp --------------------------------------------- 9 

C. santamaria (Tucker) ----------------------------------------------------------- 11 

C. middlesexensis (Mansfield) ------------------------------------------------------ 12 

C. jeffersonius (S.ay) ------------------------------------------------------------- 13 C. septenarius (Say) ____ __ _ __ _ _ ___ _ ___ _ _____ _ _____ ______ _ ____ _ ___ __ ___ _ ____ _ __ _ __ 15 

C. madisonius (Say) -------------------------------------------------------------- 16 Locality register _______________ ------------- __________________ -------- ____ __ ____ _ ___ _ __ _ __ 18 

References cited _________________________ ----- _____________________ -------- _ ____ ___ _ _ ___ _ _ 2:0 

Index ------------------------------------------------------------------------------------ 23 

PLATE 1. 

2. 

3. 

4. 

5. 

6. 

7. 

ILLUSTRATIONS 

[Plates follow index] 

Chesapecten jeffersonius (Say). 

Chesapecten jeffersonius (Say) and C. nefrens n. sp. 

Chesapecten c,occymelus (Dall), C. s.p., and C. nefrens n. sp. 

Chesapecten nefrens n. sp. and C. santamaria (Tucker). 

Chesapecten middlesexensis (Mans.field) and C. jeffersonius (Say). 

C hesapecten madisornius (Say) and C. septenarius ( Say) . 

Chesapecten madisonius, C. septenarius, C. jeffersonius, C. middlesexensis, C. santamaria, C. nefrens, and C. coccymelus. 

Pasre 

FIGURE 1. Map showing localities where specimens of Chesapecten were obtained in Virginia, 

Maryland, and North Carolina ---------------------------------------------- 2: 

2. Stratigraphic succession of Chesapecten ------------------------------------------ 5 

TABLE 1. 

2. 

3. 

TABLES 

Specific and subs.pecific names applied to Miocene and Pliocene species of Chesapecten -

Stratigraphic succe·ssion of different species of Chesapecten in Maryland and Virginia 

Morphological feature abbreviations us.ed in the measurement of specimens ---------

Page 

4 

4 

9 

III 

CHESAPECTEN, A NEW GENUS OF PECTINIDAE 

(MOLLUSCA: BIVALVIA) FROM THE MIOCENE AND PLIOCENE 

OF EASTERN NORTH AMERICA 

By LAUCK W. WARD and BLAKE "\V. BLACKWELDER 

ABSTRACT 

The genus Chesapecten (type species, Chesapec,ten nefrens 

Ward and Blackwelder n. sp.) is proposed for an important 

group of eastern North American Pectinid.ae. Chesapecten 

species are distinguished by their strong ribbing, their large 

size (adul,ts often longer than 130 mm), the extreme reduction of cardinal crura, and the scabrous sculpture which 

covers the entire exterior of both valves. Included in Chesapecten are the species Pecten ( Chlamys) coccymelus Dall 

1898, Chesapecten nefrens Ward and Blackwelder n. sp., 

Chlamys (Lyropecten) santamaria Tucker 1934, Pecten 

( Chlamys) santamaria middlesexensis Mansfi..eld 1936, Pecten 

jeffersonius Say 1824, Pecten septenarius Say 1824, and 

Pecten madisonius Say 1824. Type specimens of these species 

are illustrated, including Say's specimens from the British 

Museum. Say's 1824 publication entitled "An account of the 

fossil shells of Maryland" is shown to be based primarily 

upon sp.ecimens from the Yorktown Formation in Virginia. 

Pecten madisonius is regarded as a senior synonym of the 

North Carolina and Virginia upper Yorktown species, Pecten 

edgecombensis Conrad 1862. In 1687 one member of the 

Chesapecten group was described and figured by Martin 

Lister in his "Historiae Conchyliorum, Liber III" (pL 16,7); 

this Chesapecten is the earliest illustrated and descibed 

American fossil. 

INTRODUCTION 

A distinctive morphological complex of pectinid 

species occurs in Miocene and Pliocene deposits of 

the Atlantic Coastal Plain; the name Chesapecten 

is proposed for these forms. The genus is distinguished from other taxa in that its right valve is 

flatter than the left, the cardinal crura are extremely reduced, and the entire exterior of the valves 

is covered with strong ribs and rows of scabrous 

lirae. Chesapecten 1ived with the flatter right valve 

resting on the sediment in water depths of a few 

meters to depths of more than 40 m (according to 

the biologic and lithologic characteristics of the 

deposits). The oldest species assigned to Chesapecten are found near the lower Miocene-middle 

Miocene boundary; in the upper part of the lower 

Pliocene (3 to 4 million years ago), Chesapecten 

apparently became extinct. 

Miocene and Pliocene marine deposits are exposed discontinuously from New Jersey to Florida 

in the Atlantic Coastal Plain. Historically, formational names applied to these deposits more often 

than not have been chosen on the basis of the contained invertebrate fauna and not strictly upon 

lithostratigraphic evidence. That is, formational 

names have been extended from type areas faunally rather than lithicaUy. This has been a common 

procedure at one time or another virtuaBy throughout the Atlantic and Gulf Coastal provinces, but 

the practice has lingered in the Atlantic Coastal 

Plain (Waller, 1969, p. 90-91; Murray, 1961). 

A comparison of the correlation charts of Olsson 

(1917), Clark and Miller (1912), and Mansfield 

(1928, 1936, and 1943) indicates the difficulties 

involved in using present formational terminology 

in the study area (chiefly Maryland, Virginia, and 

North Carolina) . A particular problem is the St. 

Marys Formation of Virginia. Mansfield (1936) 

considered part of his St. Marys Formation in Virginia to be the same age as the St. Marys Formation (of Shattuck, 1904) in Maryland and part to 

be younger. A very few localities in Virginia do 

contain beds which correlate with the St. Marys 

Formation of Maryland, but Mansfield did not 

recognize these beds as distinct from the rest of his 

formation, which is younger and separated from 

these beds by an unconformity. Pending revision of 

formational terminology, we use "Virginia St. 

Marys Formation" as an informal name for beds 

entirely younger than the St. Marys Formation of 

Shattuck. Within this Virginia St. Marys Formation we recognize upper and lower units which are 

separated by a distinct unconformity (as seen at 

Cobham Wharf, Va .. at 5 feet (1.5 m) above beach 

level (fig. 1, loc. 42). Caution is called for when using a formational name in Virginia because most 

workers have been inconsistent in their application of names. Consequently, in this paper strati1 
2 MIOCENE AND PLIOCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

Pamlico Sound 

20 MILES 

I 

FIGURE 1.-Localities where s.pecimens of Chesapecten were obtained in Virginia, Maryland, and North Carolina. Numbers 

keyed to the locality register. 
SUMMARY OF PAST WORK 3 

graphic names are intended to be applied only to 

the beds at localities discussed under "Occurrence" 

in the systematic section. Figure 1 shows localities 

where specimens were obtained in our sampling 

area. Numbers in the figure are keyed to the locality 

register. In addition, the locality register a1so contains numbers for localities which have been recorded in the U.S. Geological Survey Cenozoic Locafity Register. Figured specimens which have been 

collected by the writers have received U.S. National 

Museum numbers (USNM) and have been placed in 

collections of the National Museum. Chesapecten 

occurrences outside the principal study area are 

noted from other National Museum collections and 

are mentioned in the discussion on "Occurrence" 

in the systematic section. 

Chesapecten is found abundantly within formations belonging to the Chesapeake Group. These 

formations include the Calvert and Choptank Formations (of Shattuck, 1904) as seen in Maryland, 

the St. Marys Formation (of Shattuck, 1904) in 

Maryland and northern Virginia, the lower and 

upper Virginia St. Marys Formation (of Mansfield (in part), 1936) in Virginia and North Carolina, and the lower and upper Yorktown Formation (of Mansfield, 1936) in Virginia and North 

Carolina. Chesapecten is not often found in ageequivalent deposits elsewhere in the coastal plain. 

However, localities from New Jersey to Florida 

where it does occur are noted in the systematic 

section under occurrences. 

ACKNOWLEDGMENTS 

We wish to express our sincere thanks to J. E. 

Hazel, who has given generously of his time and 

experience. Druid Wilson provided much guidance, 

critically reviewed the manuscript, and loaned a 

number of collections for study. T. R. Waller, John 

Pojeta, Jr., and T. G. Gibson reviewed the manuscript and offered many valuable suggestions. We 

are indebted to C. P. Nuttall and D. L. F. Sealy 

of the British Museum and to C. R. Givens, E. A. 

Shapiro, and H. G. Richards of the Philadelphia 

Academy of Sciences for the loan of their specimens. Thor Hansen, G. B. Lawrence, and J. B. 

Green, Jr., aided in fieldwork. 

SUMMARY OF PAST WORK 

Most workers have placed the species here assigned to Chesapecten in the genus Lyropecten of 

Conrad (1863). Lyt·opecten, an eastern Pacific 

genus, is a similar but distinct early Miocene to 

Holocene genus possessing prominent cardinal 

crura; in Chesapecten the cardinal crura are extremely reduced. Such differences had been previously recognized by Mansfield (1936) and by Hertlein and Grant (1972). 

Description of the different species of Chesapecten has had a long and complex history. In 1687 

one member of the Chesapecten group was described and figured, but not named, by Martin 

Lister in his "Historiae Conchyliorum, Liber III" 

(pl. 167). With the figured specimen is the notation "A virg Ind Occ." The original label on the 

specimen probably read "a virg" and was wrongly construed by Lister to mean Virgin Islands 

(hence the "Ind Occ" meaning Indies Occidentale 

or West Indies). In 1824, Thomas Say, an American naturalist at the Philadelphia Academy of Science, recognized that Lister's plate 167 was intended to represent an American Atlantic Coast 

Tertiary bivalve species; Say (1824, p. 133-134, 

pl. 9, fig. 1) named this species Pecten jeffersonius. 

Thus, the source of Lister's specimen is thought 

to be the United States, specificaLly Virginia; there 

is no record of such a pectinid, fossil or living, 

from the Virgin Islands. 

Also, another fossil figured by Lister (1688), 

no locality or description given, is apparently a 

specimen of the distinctive bivalve M ercenaria tridacnoides (Lamarck, 1818), which commonly occurs in the same general area as Chesapecten jeffersonius in southeastern Virginia. 

Lister's description of C. jeffersonius preceded 

his illustration of another fossil said to be from 

Maryland. This taxon, the gastropod Ecphora, has 

generally been considered the first figured American fossil (Shattuck, 1904; Vokes, 1957). However, the figure of Ecphora was published in an appendix to "Historiae Conchyliorum" in 1692 and not 

the date 1685 cited by others. It seems clear, therefore, that the earliest illustrated and described 

American fossil is the species later named Chesapecten jeffersonius (Say, 1824) from the Yorktown 

Formation (lower Pliocene) of Virginia. A translation of Lister's original description is included in 

the systematic section along with notes written by 

Lister in a copy of his work. Lister's original figure 

is shown on our plate 1. 

In addition .to Pecten jefjersonius, Thomas Say 

(1824) also described two other species, here 

placed in Chesapecten. Most of the fossils described 

by Say at this time had been loaned to him by 

John Finch, a Scottish visitor to the United States. 

These fossils were mistakenly attributed by Say to 

Miocene deposits on the St. Marys River, Md. It is 

apparent from Say's descriptions, illustrations, and 
4 MIOCENE AND PLIOCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

material that he had no Maryland collections in his 

possession at the Philadelphia Academy at this 

time. Finch's description (1833) of his own travels 

in America indicates that he probably shipped all 

the Maryland material he collected directly to England from one of the ports in Virginia. The materials which Say examined at the Philadelphia 

Academy of Sciences were probably collected on 

Finch's visit to the James River near City Point 

and the York River at Yorktown (Finch, 1833, p. 

266-275). The mollusks are all indicative of the 

Yorktown Formation of southeastern Virginia and 

northern North Carolina. The three Chesapecten 

species which Say described are an valid and all 

known to occur in southeastern Virginia ( Chesapecten jefje'!'sonius, C. septenarius, and C. madisonius). Maryland specimens of Chesapecten which 

have been referred to C. madisonius by many 

authors are not this species, but represent an unnamed species described in this paper as Chesapecten nefrens. 

STRATIGRAPHIC SUCCESSION OF 

CHESAPECTEN 

Table 1 lists the previously named species here 

assigned to Chesapecten. Their stratigraphic succession is given in table 2 and is shown in figure 2. 

TABLE 1.-Specific, and subspecific names applied to Miocene 

and Pliocene species of Ghesapecten 

PreviotJ,s names This study 

septenarius Say, 1824 unchanged, locality not Maryland 

madisonius Say, 1824 [not Conrad, unchanged, locality not Maryland 

1840; Glenn, 190~; Mansfield, 1936; 

Tucker-Roland, 1938, Schoonover, 

1941; Gardner, 1943] 

jeffersonius Say, 1824 unchanged, locality not Maryland 

madison ius (of Conrad, 1840, p. 48, nefrens, new species 

pl. 24, fig. 1; Glenn, 1904, p. 377, 

pl. C, fig. 1; Gardner, 1943, pl. 

4, fig. 5) 

edgecombensis Conrad, 1862, and madisonius Say, 1824 

others 

aayanus Dall, 1898 unchanged 

coccymelus Da!l, 1898 redefined 

santamaria Tucker, 1934 unchanged 

santamaria middlesexensis Mansfield, m.iddlcsexensis (Mansfield) 

1936 

jeffersonius palmyraensis Mansfield, septenarius Say 

1936 

madisonius bassleri Tucker-Roland, coccymelus Dall 

1938 

TABLE 2.-Stratigraphic succession of different species of Chesapecten in Maryland and Virginia 

Zone 2 2 Yorktown 

Formation ------ Zone 1 2 Yorktown 

FtOrmation ------ "upper Virginia St. 

Marys Forma;tion_ 

"lower Virginia St. 

Marys Formation_ 

St. Marys Formation, 

Windmill Point, 

Md ------------- St. Marys Formation, 

Langley Bluff, Md_ 

St. Marys Formation, 

Little Cove Point, 

Md ------------- Ghoptank Formation 

Zone 20 3 --------

Ghoptank Formation 

Zone 19 3 --------

Ghoptank Formation 

Zone 18 3 _______ _ 

Ghoptank Formation 

Zone 17 3 _______ _ 

Ghoptank Formation 

Zone 16 3 _______ _ 

Calve.rt Formation 

Zone 14 3 _______ _ 

Oalve,rt Formation 

Zone 10 3 _______ _ 

C. coccymelus 

X 

1 Schoonover, 1941, pl. 5, fig. 1. 2 Mansfield, 1943. 3 Shattuck, 1904. 

C. sp. 1 

X 

C. nefrens 

X 

X 

X 

X 

X 

X 

X 

C. nefrens- I 

C. santamaria 

(transitional C. santa maria 

form) 

X 

X 

C. middlesexensis 

X 

X 

C. jeffersonius 

X 

C. septenarius 

X 

C. madisonius 

X 
Chesapecten 

jefferson ius 

Chesapecten 

septenarius 

VIRGINIA ST. MARYS 

FORMATION 

MARYLAND ST. MARYS 

FORMATION 

ZONES 16-20 

CHOPTANK FORMATION 

ZONE 14 

CALVERT FORMATION 

ZONE 10 

CALVERT FORMATION 

STRATIGRAPHIC SUCCESSION 

ZONE 2 

YORKTOWN FORMATION 

Chesapecten 

madison ius 

ZONE 1 

YORKTOWN FORMATION 

Chesapecten middlesex·ensis 

(transitional to C. jeffersonius) 

Chesapecten santamaria 

Chesapecten nefrens 

Chesapecten coccym,elus Chesapecten sp. 

FIGURE 2.-Stratigraphic succession of Chesapecten. 

5 
6 MIOCENE AND PL.WCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

As indicated on table 2, species ranges seldom extend across important stratigraphic breaks. Near 

these stratigraphic breaks some transitional forms 

do appear within populations containing specimens 

morphologically more typical of the species as a 

whole. However, no distinct morphological discontinuities appear to exist until stratigraphic breaks. 

For these reasons Chesapecten is an important 

stratigraphic guide in the Chesapeake Group and 

possibly elsewhere. Reference localities of particular 

species occurrences are listed in the systematic section. 

Eocene, Oligocene, and earliest Miocene pectinids 

are small in size and differ from Chesapecten in the 

nature of the exterior sculpture and auricles. Apparently the first species of Chesapecten occur in the 

lower part of the middle Miocene Calvert Formation (Zone 10 of Shattuck, 1904). This interval 

(Zone 10) has been placed near the lower and middle Miocene boundary (Zones 8 and 9 of Berggren, 

1972) on the basis of planktonic foraminifers 

(Gibson, 1971). This early species is Chesapecten 

coccymelus (Dall, 1898), a relatively small pecten 

with quite scaly sculpture. Occurring with C. 

coccymelus is a similar but as yet poorly understood species which lacks the pronounced scaliness. 

These two early species are succeeded by Chesapecten nej1·ens n. sp., which is herein designated the 

type species of the genus. Chesapecten nefrens is 

found in the Calvert Formation and in the middle 

Miocene Choptank Formation (Zones 14 to 20 of 

Shattuck, 1904) of Maryland. A younger form, 

possibly a subspecies of Chesapecten nefrens also, 

occurs in the probable upper Miocene St. Marys 

Formation at Little Cove Point (fig. 1, loc. 10). 

Chesapecten santamwda (Tucker, 1934) is apparently derived from C. nefrens; it is similar in 

valve outline, convexity, nature of byssal notch, 

valve gape, coarseness of ctenolium, depressed broad 

byssal fasciole, number of ribs, shell thickness, and 

size, but differs in its finer, more uniform scabrous 

sculpture, its broader, flatter ribs, and its smaller 

hinge area. The earliest C. santamaria is represented by a morphologically intermediate form 

found at the well-known Langleys Bluff collecting 

site (fig. 1, loc. 11) in the St. Marys Formation, 

Maryland. Chesapecten santamaria occurs in the 

Miocene St. Marys Formation in Maryland at Windmill Point (loc. 12), St. Marys River, and at a 

number of localities in Virginia. As previously 

mentioned, beds in Virginia which have been 

termed part of the St. Marys Formation are altogether younger (probably of latest Miocene age) 

than that part of the St. Marys Formation in which 

C. santamaria is found. This age relationship is 

clear, according to our present investigations, from 

both the nature of the physical stratigraphy and 

the nature of the molluscan assemblages. In this 

paper these younger parts of the St. Marys are informally referred to as the "lower Virginia St. 

Marys Formation" and "upper Virginia St. Marys 

Formation." 

Chesapecten middlesexensis (Mansfield, 1936) occurs in both the lower and the upper Virginia St. 

Marys Formation. It is also reported from the Red 

Bay Formation of Florida (Druid Wilson, U.S. 

Geological Survey, oral commun., 1973). It is a 

distinct species from C. santamaria and can be 

recognized by its greater inflation, wider auricles, 

shallower byssal notch, and heavier valves. 

In the upper beds of the Virginia St. Marys 

Formation are forms gradational between C. middlesexensis and its evolutionary successor, C. jeffersonius (Say, 1824), which has more inflated 

valves, a very shallow byssal notch, and a very 

heavy sheJl. Chesapecten jeffersonius is confined to 

Zone 1 of Mansfield (1943) in the Yorktown Formation. This species has been found as far north as 

New Jersey (in shallow dredging) and occurs in 

parts of the Tamiami Formation in Florida. 

Chesapecten madisonius (Say, 1824) and Chesapecten septenarius (Say, 1824) succeed C. jefje?·-

sonius and are found in Zone 2 (Mansfield, 1943) of 

the Yorktown Formation and in deposits as far 

south as southern Florida. Chesapecten madisonius 

differs from C. jeffersonius in being much less convex, and it has a much more profound sinus in the 

ear of the superior valve. The strength of the ribs 

is much reduced, and the ribs are obscured on the 

interior of the shell by thick layers of calcite. The 

whole surface is covered by scaly striae. 

Chesapecten septena1·ius differs from C. jeffet·-

sonius in having fewer primary ribs; also, its ribs 

are broader and usually stronger. Chesapecten septenari~~s and C. madisonius represent the end or 

near end of the Chesapecten lineage; the group becomes extinct during late Yorktown time (upper 

part of the Orionina vaughani Zone of Hazel, 1971), 

or apparently about 3 to 4 million years ago (Akers, 

1972; Berggren, 1972, Joseph Hazel, U.S. Geol. Survey, oral commun., 1974) 

REMARKS 

Chesapecten is an important genus in the Atlantic 

Coastal Plain, in both a stratigraphic and a historic 

sense. The fact that a proper generic status had not 
SYST·EMATIC DESCRIPTIONS 7 

been accorded this group and that only informal 

stratigraphic terminology is availab1e for many of 

the units involved demonstrates the extreme need 

for competent- biostratigraphic work in these Tertiary units. 

The species recognized in this paper are the 

dominant Chesapecten in the Atlantic Coast Cenozoic. The species as presently defined are excellent 

stratigraphic guides to the major geologic events in 

the Chesapeake Group and can be easily recognized 

in the field. It is possible to determine finer taxonomic divisions within some species of the Chesapecten group, but from our observations it is 

thought that these forms should probably have subspecific or informa~l rank. We have not named subspecies in this paper, feeling instead that such subdivision should be done as a separate study which 

would relate very small population variation to 

sediment parameters and to other physical and biological parameters; such a study is beyond the 

scope of the present paper. In certain species there 

appear to be transitional forms from one species 

to the next (as in nefrens-santamaria, middlesexensis-jefjersonius, and jefjersonius-madisonius). That 

is, in the later part of some species ranges, certain 

individuals possess morphological features which 

are more characteristic of the succeeding species. 

However, major morphologic discontinuities usually do not occur until stratigraphic unconformities. 

For these reasons, we feel that the presently available specific names provide a convenient and useful division of a partly gradational Chesapecten 

sequence and we prefer to recognize the intermediaries (found in the extremes of different species 

ranges) as transitional forms. 

SYSTEMATIC DESCRIPTIONS 

Phylum MOLLUSCA 

Class BIVALVIA 

Order PTERIOIDA 

Superfamily PECTINACEA 

Family PECTINIDAE Rafinesque, 1815 

Subfamily CHLAMYDINAE Korobkov, 1960 

Genus CHESAPECTEN Ward and Blackwelder, new genus 

Type species.-Chesapecten nej?·en8 vVard and 

Blackwelder, n. sp. 

Diagnosis.-This genus is characterized by its 

generally large size, the extreme reduction of cardinal crura, the scabrous sculpture over the entire 

exterior of the valves, and the greater convexity of 

the left valve. 

Description.-Valves of adults usually longer 

than high, large (often 120 mm or more in height), 

biconvex, with the left valve more inflated than the 

right. 

Auricles well developed and approximately equal 

in length, byssal notch deep to shallow; auricular 

denticles present, posterior auricular denticles 

usually weak, trace of outer ligament broad, 

ctenolium present in juveniles but absent in adults 

of heavy-shelled species; byssal fasciole broad. 

Plicae of right valve usually 10 to 16 in number, 

but as few as 4 and as many as 23 in different 

species; plicae and exterior sculpture usually 

equally strong on both valves; rows of scabrous 

lirae over entire exterior of the valves ; width of 

plicae about equal to that of interspaces. 

Resilial insertions deep, higher than long, 

bounded by raised protuberances in the right valve; 

in the left valve the resilial insertion extends into 

the interior on a platform for about a distance of 

one-third the height of the resilial insertion; in the 

right valve the extension is much reduced; cardinal 

crura obsolete; adductor insertions large with a 

distinct nonstriate part usually about half the size 

of the striate part. 

Remarks and cmnparisons.-Chesapecten differs 

from Chlamys in having relatively few ribs; also, 

its ribs are usually heavy and not dichotomous. 

Adult Chesapecten are also usually longer than 

high. In Chesapecten, unlike M acrochlamys and 

Lyropecten, the cardinal crura are extremely reduced or absent. Chesapecten occurs in middle Miocene to lower Pliocene Atlantic Coastal Plain strata 

from New Jersey to Florida. 

Many of the features which are used to characterize different groups within the Chlamydinae 

are not constant within Chesapecten. The byssal 

notch changes within the evolutionary history of 

the group, as do valve convexity and the nature of 

the free margins of the posterior auricles. Many of 

the changes are probably adaptations to an unattached sedentary life habit, like that of Chesapecten je.({erson1~us. Earlier members of this genus, 

such as Chesapecten nefrens (pl. 2), were lighter 

shelled, free swimming or byssally attached species. 

Ontogenetic variation in Chesapecten may be described as follows: 

1. The length to height ratio changes, with the 

juveniles tending to be higher than long or 

equidimensioned whereas adults are longer 

than high. 

2. Auricular denticles diminish or disappear. 

3. In C. middlese~rensis and all succeeding species 

the byssal notch becomes reduced with age, 

while C. nefrens and C. santamaria tend to 

to retain the byssal notch as adults. 

4. Internal deposits of calcite increase with shell 
8 MIOCENE AND PLIOCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

size, especially in C. 1niddlesexensis and all 

succeeding species. 

5. Scaling may become divergent in adult forms, 

especially in C. madisonius and in C. middlesexensis. 

6. In C. jejje1·sonius the smaller individuals have 

very square-shaped ribs, and these ribs become rounded as the shell becomes larger. 

This trend is carried on to some extent in all 

species. 

The name Chesapecten is used for this group because the development of this genus is best seen in 

exposures in the Chesapeake Bay and in adjoining 

rivers. 

Cbesapecten coccymelus (Dall) 

Plate 3, figures 1, 2; plate 7, figures 14, 15 

1898. Pecten (Chlamys) coccynwlus Dall, p. 741-742, pl. 34, 

fig. 1. 

1904. Pecten ( Chla.mys) coccymelus Dall. Glenn, p. 37 4-375, 

pl. 99, fig. 3. 

1936. Chlamys (Chlamys) coccymelus (Dall). Rowland, p. 

1007-1008, pl. 8, fig. 3, 4. 

1938. Chlamys (Lyropecten) maclisonius bassleri TuckerRowland, p. 13-14, pl. 5, fig. 1. 

Original description.-Shell small, ovate, inflated, strongly sculptured, with unequal ears; 

disk with eighteen narrow, high, compressed ribs, 

with wider interspaces, which near the basal margin carry one or two very small radial threads ; 

the backs of the ribs support numerous high, evenly spaced, distaHy guttered, small spines ; in the 

interspaces only transverse sculpture of wavy incremental lines; submargins small, narrow, with 

fine, beaded radial threads, which in the left valve 

also extend over the ears; hinge-line short, the 

cardinal crura developed, sharply cross-striated; 

auricular crura present; interior of the disk fluted 

in harmony with the external ribs. Alt. 30, lat. 25, 

semidiam. 5 mm. 

Diagnosis.-Chesapecten coccymelus is a small 

Chesapecten, usually less than 60 mm in height, 

having very high distally concave spines and unequal auricles, the anterior auricle being longer than 

the posterior. 

Description.-Valves small, usually less than 60 

mm in height, thin, suborbicular, biconvex, left 

valve inflated, right valve less inflated, small gapes 

present along ventral commissure owing to inexact 

match of opposing valves; small gapes between opposing disk flanks. 

Auricles well deve~loped, anterior auricle larger 

than posterior, auricular denticles weakly expressed 

and in contact when valves together; free margins 

of posterior auricles flat in plane of commissure; 

byssal notch deep, ctenolium coarse, byssal fasciole 

broad and depressed. 

Plicae of right valve number 15 to 19, each plica 

strongly sculptured with one to three rows of very 

high distally concave spines and with one to three 

rows of less elevated spines in the interspaces. 

Trace of outer ligament relatively narrow, resilial insertion not deep, higher than long, extending 

into the interior for about one-third of its height in 

the left and right valves, auricular denticles reduced, cardinal crura reduced (especially in larger 

specimens), ribbing expressed over most of interior of shell ; dorsal margin encroaching on ligament. 

Illust1·ated specimens and measurements.-Holotype, USNM 87754 left valve from Plum Point, Md. 

(Calvert Formation) (USGS loc. 25294). Length 

24.3 mm, height 30.2 mm, total length of auricles 

along line of ligament 14.2 mm, length of auricle 

anterior to center of resilial insertion 9.0 mm, 

angle formed by lines through origin of shell 

growth (at beak) and (both anteriorly and posteriorly) tangent to, but nowhere intersecting, edge 

of disk, 92°. Convexity of left valve 5.4 mm (pl. 

3, fig. 1; pl. 7, fig. 15). 

USNM 193440 left valve from 0.5 mile (0.8 km) 

below Camp Roosevelt, Md. (Zone 10, Calvert Formation) (USGS loc. 25295). Length 38.0 mm, 

height 39.2 mm, total length of auricles along line 

of ligament 25.0 mm, length of auricle anterior to 

center of resilial insertion 14.4 mm, angle formed 

by lines through origin of shell growth (at beak) 

and (both anteriorly and posteriorly) tangent to, 

but nowhere intersecting, edge of disk, 103°. Convexity of left valve 8.1 mm (pl. 3, fig. 2). 

USNM 207692, right valve, (USGS loc. 25295), 

length of auricles along line of ligament 14.2 mm 

(pl. 7, fig. 14.). 

The following measurements (in millimeters) 

have been taken from the right valves of additional 

specimens of Chesapecten coccymelus from map locality 3 (USGS loc. 25295) (see table 3 for explanation of the abbreviations) : 

Specimen LN HT LA LAA ANG ( 0 ) CON RIB 

1 57.4 58.1 34.5 19.6 110 9.4 16 

2 48.8 49.8 30.3 18.8 101 9.9 17 

3 45.8 46.7 27.0 16.1 102 8.1 15 

A 44.8 46.5 2n.4 14.9 101 8.4 17 

5 43.5 44.8 25.3 15.7 102 7.2 17 

6 42.!l 44.1 23.4 14.2 99 8.0 18 

7 41.6 41.6 25.2 14.3 106 7.3 17 

8 34.6 3H.2 21.0 12.5 95 5.7 16 

9 30.3 31.9 19.5 12.1 96 5.1 17 

10 26.4 29.1 17.7 11.0 94 5.1 17 

11 22.0 24.7 15.1 8.8 90 3.2 15 

12 17.1 19.8 12.7 7.2 85 2.9 17 
SYSTEMATIC DESCRIPTIONS 9 

TABLE 3.-M orphological feature abbreviations used in 

measurement of specimens 

LN Length 

HT Height 

LA Total length of auricles along line of ligament 

LAA Length of auricle anterior to center of resilial insertion 

ANG Angle formed by lines through origin of shell growth 

(at beak) and (both anteriorly and posteriorly) 

tangent to, but nowhe,re intersecting edge of disk 

CON Convexity of valve 

RIB Number of ribs 

Occurrence.-The type locality is the Calvert 

Formation, Plum Point, Md. Additional localities 

are Zone 10 (Shattuck, 1904) of the Calvert Formation from Randall Cliff (fig. 1, loc. 1; USGS loc. 

25293) south to Plum Point (fig. 1, loc. 2, USGS 

loc. 25294) . 

Comparison and remarks.-Chesapecten coccymelus differs from C. nefrens in its smaller size 

(average adults being only 40 to 50 mm long as 

opposed to 150 mm for C. nefrens), its rows of 

very high distally concave spines, and its unequal 

auricle size, the anterior auricle being longer than 

the posterior. 

Dall's original specimen is an end member of a 

somewhat variable species. We have provided a 

new description which includes the more morphologically typical specimens of this species. A 

comparison of the holotype (pl. 3, fig. 1) and our 

specimen (pl. 3, fig. 2) shows that the number of 

rows of spines is a variable character. Contrary to 

Dall's original description, we consider the cardinal 

crura in this species to be reduced, especially in 

adult specimens. This species is common in Zone 

10 of the Calvert Formation. 

Mongin (1959) suggests that a new name, calvertensis, be applied to several species, including 

C. bassleri (Tucker-Rowland). The name calvertensis does not have priority and cannot be used. 

The species intended to be grouped under this name 

do not appear to be closely related. 

Chesapecten nefrens Ward and Blackwelder, n. sp. 

Plate 2, figures 4-6; plate 3, figures 4-7; plate 4, figures 1, 2; 

plate 7, figures 6, 13 

1840. Pecten Madisonius Say. Conrad, p. 48, pl. 24, fig. 1. 

1863. Pecten Madisonius Say. Conrad, p. 581. 

1894. Pecten M adisonius Say. Whitfield, p. 30, pl. 4, fig. 5. 

1904. Pecten (Chlamys) madisonius Say. Glenn, p. 377, pl. 

C,fig. 1. 

1909. Pecten ( Chlamys) madt'som'us Say. Grabau and 

Shimer, p. 502, fig. 673C. 

1936. Pecten ( Chlamys) madt'sonius Say. Mansfield, p. 17 4, 

176, 177, 184. 

1938. C hlamys ( Lyropecten) madisonia (Say). TuckerRowland, p. 9-11, pl. 1, fig. 1, 2. 

1941. Chlamys (Lyropecten) madisonius (Say). Schoonover, 

p. 28-37, pl. 3, figs. 1-3; pl. 4, fig. 4; pl. 5, fig. 3. 

1943. Chlamys (Lyropecten) madisonia (Say) Glenn. Gardner, p. 32, pl. 4, fig. 5; pl. 9, fig. 7. 

1959. Chlamys madisonia (Say). Mongin, p. 309-314, pl. 26, 

figs. la-b. 

Diagnosis.-Chesapecten nefrens is a large Chesapecten with large gapes between disk flanks and 

along the ventral commissure, nonuniform scabrous 

lirae, a pronounced byssal notch, unflattened ribs, 

and an only slightly convex right valve. 

Description.-Valves large, adults often greater 

than 120 mm in height, suborbicular, biconvex, left 

valve strongly inflated, right valve very slightly inflated, most adults longer than high; gapes present 

along the ventral commissure owing to inexact 

match of opposing valves; very large gapes between 

opposing disk flanks. 

Auricles well developed, subequal; auricular 

denticles present but not in contact when valves 

together. Free margins of posterior auricles flat and 

gaping in plane of comissure. Byssal notch deep, 

ctenolium coarse, but obscured in some adults; 

byssal fasciole broad, and very depressed. 

Plicae of right valve usually number 13 to 16, 

each plicae bears three scabrous lirae; other lirae, 

less elevated than those upon the summits of the 

plicae, crowd the sides and interspaces. 

Trace of outer ligament broad in adults, resilial 

insertion fairly deep, almost twice as high as long; 

resilial insertion extends into the interior for about 

one-third of its height in the left valve and slightly less in the right valve, posterior auricular denticles reduced, anterior auricular denticles moderately expressed, cardinal crura obsolete, pallial line 

weakly expressed; ribbing expressed over most of 

interior of shell. Adductor insertions large with 

distinct nonstriate portion less than half the size 

of the striate portion; dorsal margin often encroaches on ligament, gill suspensor insertion not 

impressed. 

Illust1·ated specimens and measurements.-Holotype, USNM 199442 paired valves from Calvert 

County, Md., Chesapeake Bay Cliffs zone 19 (of 

Shattuck) at Camp Conoy 8 ft (2.5 m) above 

beach (USGS loc. 25299). Length 177.4 mm, height 

158.8 mm, total length auricles along line of ligament 101.5 mm, length of auricle anterior to center 

of resilial insertion 46.8 mm, angle formed by lines 

through origin of shell growth (at beak) and (both 

anteriorly and posteriorly) tangent to, but nowhere 

intersecting, edge of disk, 128°. Convexity of left 

valve 40.7 mm. Convexity of right valve 27.6 mm 

. (pl. 3, fig. 4-7). 

Paratype, USNM 193443 paired valves from Calvert County, Md., Chesapeake Bay Cliffs Zone 19 
10 MIOCENE AND PLIOCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

at Camp Conoy at 8 ft (2.5 m) above beach 

(USGS loc. 25299). Length 134.3 mm, height 113.9 

mm, total length auricles along line of ligament 

73.2 mm, length of auricle anterior to center of 

resilial insertion 38.6 mm, angle formed by lines 

through origin of shell growth (at beak) and (both 

anteriorly and posteriorly) tangent to, but nowhere 

intersecting, edge of disk 131 o. Convexity of left 

valve 30.8 mm. Convexity of right valve 12.4 mm 

(pl. 2, figs. 4-6; pl. 4, figs. 1, 2; pl. 7, figs. 6, 13). 

The following measurements (in millimeters) 

have been taken from the right valves of additional 

specimens of Chesapecten nej1·ens from map locality 

7 (USGS loc. 25299) (see table 3 for explanation of 

the abbreviations) : 

Specimen LN HT LA LAA ANG ( 0

) CON RIB 

1 107.6 100.5 60.5 30.5 116 10.3 13 2 106.8 96.2 62.4 31.4 122 9.7 14 a 104.4 98.:3 55.5 28.2 118 9.5 13 4 67.0 62.7 39.2 23.8 113 7.2 14 5 57.4 58.3 37.1 21.6 106 5.1 13 6 55.7 52.3 36.3 18.9 109 8.0 13 7 36.5 40.3 23.0 13.9 92 5.1 15 8 32.4 31.6 20.5 12.0 105 4.1 12 9 24.2 26.1 17.2 9.5 98 3.0 14 

Occurrence.-The type locality is the Choptank 

Formation, Zone 19 (of Shattuck, 1904) at Camp 

Conoy, Calvert County, Md. (fig. 1, loc. 7; USGS 

loc. 25299). Chesapecten nej1·ens also occurs in the 

following Maryland middle Miocene zones of 

Shattuck: 

Calvert Formation 

Zone 14 from Camp Roosevelt (fig. 1, loc. 

3) to Governor Run (fig. 1, loc. 4; 

USGS loc. 25296) 

Choptank Formation 

Zone 16 from Scientists' Cliffs (fig. 1, loc. 

5; USGS loc. 25297) to Calvert Beach 

(fig. 1, loc. 6; USGS loc. 25298) 

Zone 17 from Scientists' Cliffs to Camp 

Conoy (fig. 1, loc. 7) 

Zone 18 from Scientists' Cliffs to Camp 

Conoy 

Zone 19 from Scientists' Cliffs to Cove 

Point (fig. 1, loc. 8; USGS loc. 25306) 

(including loc. 9, USGS 25301). 

A variety of this species also occurs in the St. 

Marys Formation at Cove Point, Md. (fig. 1, loc. 

8 ; USGS loc. 25300) . 

Comparison and remarks.-Chesapecten ne/1·ens 

differs from C. santamaria in having narrower, unflattened ribs, larger gapes between disk flanks, 

larger gapes along the ventral commissure, nonuniform scabrous lirae, and a larger byssal notch. 

Most workers studying the Maryland Miocene 

have referred to this species as Pecten madisonius 

of Thomas Say ( 1824). However, as previously 

mentioned, the specimens which Thomas Say examined did not come from Maryland but from the 

Yorktown Formation in southeastern Virginia. 

John Finch, who loaned Say most of the fossils that 

Say described in 1824, requested that three species 

of his collection that might prove to be new, should 

be dedicated to three particular men. Say does name 

three species: Pecten jeffersonius, Pecten madisonius, and Pecten cl?:ntonius. In his description of P. 

madisonius, however, Say mentions that his description was taken from specimens presented to the 

academy by a Mr. Watson. Whether there were 

other specimens in Finch's collection which were 

identical to those presented by Mr. Watson is not 

known. Specimens which must have come from the 

Virginia Yorktown Formation are found in Finch's 

collection and do fit the description given by Say. 

Watson evidently purchased his specimens at the 

sale of collections of one Professor Barton. In Barton's collection is a Virginia specimen of Crassatella undulata Say known only from the Yorktown 

Formation which indicates that Barton's collection 

came from the Yorktown. Additional evidence that 

Say's P. madisonius specimens came from the Virginia Yorktown Formation is given by the statement in Schoonover (1941, p. 29) : 

The only specimen with Say's label is not C. madisonius, 

but probably C. jeffe1·sonius edgecombensis. The label indicates that the specimen came from St. Mary's River, Maryland, but this is probably a mistake, and it more likely came 

from Virginia. 

The specimen was obtained by the writers from 

the Academy of Nat ural Sciences of Philadelphia. 

The measurements and description agree almost 

exactly with that given by Say. This specimen is 

treated in this paper as the only known existing 

original type specimen of Pecten madisonius. 

It is apparent that Say's original specimens of 

Pecten madisonius were what Conrad and later 

workers have called Pecten edgecombensis (Conrad, 1862). (See table 1.) 

Despite the common usage of the name "madisonius" for most of the Maryland Chesapecten, this 

species name can no longer be considered applicable 

to these specimens. Chesapecten madisoni?JS is a 

validly described species from the Yorktown Formation of Virginia. Because many of Say's specific 

names have been applied to distinctly different 

species from Maryland, following the nomenclatural 

rules of priority and restricting names to the type 

material will best serve to promote clarity within 

the taxonomy of these Miocene and Pliocene species. 
SYSTEMATIC DESCRIPTIONS 11 

Chesapecten santamaria (Tucker) 

Plate 4, figures 3-6; plate 7, figures 5, 12 

1904. Pecten (Chlamys) jeffersonius Say. Glenn, p. 378-379, 

pl. C, fig. 2. 

1934. Chlamys (Lyropecten) santamaria Tucker, p. 615, pl. 

26, fig. 2. 

1938. Chlamys (Lyropecten) santamaria Tucker. TuckerRowland, p. 16-17, pl. 1, figs. 5, 6; pl. 2, fig. 10. 

1957. Chlamys (Lyropecten) santamaria Tucker. Vokes, p. 

11, pl. 7, fig. 7. 

1959. Chlamys santamaria Tucker. Mongin, p. 308-309. 

Original description.-Shell equilateral, suborbicular, inequivalved. Radial sculpture of 12 to 16, 

most commonly 12 to 14, broad, well elevated ribs 

wh;ch are approximately 4 mm narrower, at the 

ventral margin, than the inters paces. Right valves 

commonly have about 12 ribs which are about 2 

mm narrower than the interspaces. The right 

valves are flat, or nearly so, and resemble C. madisonius Say very closely in outline; left valve convex in the umbonal region; auricles subequal, radial 

sculpture of fine, scaly threads, the anterior the 

more coarsely threaded; fasciole broad. Auricles of 

the left valve subequal, uniformly sculptured with 

fine scaly threads. Summits and sides of the ribs 

threaded with numerous fine, scaly threads. Interspaces similarly ornamented ; one or two of the 

threads are usually more strongly developed, especially on the right valve. Submargins have a fine 

uniform sculpture of closely spaced, scaly threads. 

Interior ribbed to the umbones. Provinculum present. Ctenolium retained throughout the life of the 

individual. Dimensions: right valve, height, 130, 

width 126 mm (figured shell) ; left, height 120, 

width 115 mm; convexity 33.5 mm. 

Diagnosis.-Chesapecten santamaria has flattened ribs, a deep byssal notch, a short hinge area, 

exterior sculpture of uniform scabrous lirae, and 

an only slightly convex right valve. 

Description.-Valves large, often greater than 

120 mm in height, suborbicular, biconvex, left valve 

strongly inflated, right valve very slightly inflated, 

most adults longer than high, valves relatively thin, 

small gapes present along ventral commissure owing to imperfect match of opposing valves, moderate 

gapes between opposing disk flanks. 

Auricles well developed, subequal, byssal notch 

deep, apex of byssal notch gently curved, outer ligament rather narrow in adults, ctenolium coarse, 

byssal fasciole broad and depressed in a gentle 

U-shape. 

Plicae of right valve usually number 12 to 14 · 

plicae elevated, broad, somewhat flattened, with flat 

interspaces. Entire exterior of both valves covered 

with uniform scabrous lirae. 

Auricular denticles present and in contact with 

opposing denticles, resilial insertion deep, almost 

twice as high as long, with slightly raised protuberances on either side of the insertion in the right 

valve. In the left valve the resilial insertion extends 

into the interior on a platform for about one-third 

of its length. In the right valve the platform is almost absent. Cardinal crura obsolete, pallial line and 

most muscle insertions only vaguely impressed. Ribbing expressed over most of shell interior. Adductor 

insertions moderately large. 

Illustrated specimens and measurements.-Lectotype, USNM 193448 (paratype of Tucker, 1934) 

right valve from St. Marys River, Md. (St. Marys 

Formation) (USGS loc. 2342). Length (broken), 

height 124.7 mm, total length auricles along line of 

ligament 59.0 mm, length of auricle anterior to 

center of resilial insertion 33.0 mm, angle formed by 

lines through origin of shell growth (at beak) and 

(both anteriorly and posteriorly) tangent to, but 

nowhere intersecting, edge of disk (broken). Convexity of right valve 14.6 mm (pl 4, fig. 3). 

Paratype USNM 2498 (cotype of Tucker, 1934) 

left valve from St. Marys River, Md. (St. Marys 

Formation). Length 123.6 mm, height 122.5 mm, 

total length auricles along line of ligament 55.0 mm, 

length of auricle anterior to center of resilial insertion 28.5 mm, angle formed by lines through origin 

of shell growth (at beak) and (both anteriorly and 

posteriorly) tangent to, but nowhere intersecting, 

edge of disk, 123°. Convexity of left valve 27.1 mm 

(pl. 4, fig. 4) 0 

USNM 193444 paired valves from Essex Mill, 3.5 

miles (5.5 km) south of Tappahannock, Va. (USGS 

loc. 25058). Length 129.0 mm, height 121.8 mm, 

total length of auricles along line of ligament 54.5 

mm, length of auricle anterior to center of resilial 

insertion 30.5 mm, angle formed by lines through 

origin of shell growth (at beak) and (both anteriorly and posteriorly) tangent to, but nowhere intersecting, edge of disk, 121 o. Convexity of right valve 

15.0 mm (pl. 4, figs. 5, 6; pl. 7, figs. 5, 12). 

The following measurements (in millimeters) 

have been taken from the right valves of additional 

specimens of Chesapecten sa.ntamaria from fig. 1, 

locality 12 (USGS 25304) (see table 3 for explanation of abbreviations) : 

Specimen LN HT LA LAA ANG ( 0 ) CON RIB 

1 144.9 132.:~ 61.9 32.7 126 21.4 12 

2 138.5 129.7 60.3 31.8 120 17.1 11 

3 125.7 111.:~ 56.4 30.5 124 15.4 12 

4 124.9 110.0 52.1 29.4 129 12.8 14 

121.9 115.1'( 53.4 30.7 115 14.1 14 

58.5 59.2 (1) (1) 111 6.9 14 

35.4 34.9 23.6 13.8 105 4.1 15 

30.5 31.1 17.6 10.2 102 4.3 14 

1 Broken 
12 MIOCENE AND PL,IOCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

Occurrence.-Original type locality was given as 

the St. Marys River, Md. As a reference locality in 

the type area we designate the tan shell bed (3 ft 

(1m) above high tide) in the St. Marys Formation 

at Windmill Point, St. Marys River, St. Marys 

County, Md. (fig. 1, loc. 12; USGS loc. 25304). This 

locality is probably the source of the type material. 

Additional reference localities (in the St. Marys 

Formation of Shattuck 1904) are Essex Mill, Va. 

(fig. 1, loc. 13; USGS loc. 25058), White Oak Landing, Va. (fig. 1, loc. 14; USGS loc. 25059), and 

Mantua Landing, Va. (fig. 1, loc. 15; USGS loc. 

25061). 

Comparison and re1narks.- Chesapecten santamaria differs from C. rniddlesexensis in having a 

less inflated right valve, a deeper byssal notch, flatter ribs which are squared off, less coarse subradials 

on the ribs, longer retention of the ctenolium, and a 

shorter hinge area. 

Contrary to Tucker's description, the plicae in C. 

santamaria are not usuaHy narrower than the interspaces but subequal and slightly variable in proportion from specimen to specimen. Also specimens 

from Langleys Bluff do not appear to be the same 

species but rather are intermediate between C. santamaria and C. nefrens. Tucker's (1934) measurements in the description of this species are incorrect. 

In naming C. santamaria, Tucker (1934) designates two specimens as cotypes and one specimen as 

a paratype. The cotype specimen, which was figured, 

was stated to be in the collection of the Paleontological Research Institution, Ithaca, N.Y. However, it 

is apparently not there (Brann and Kent, 1960). The 

other cotype (a left valve) and the paratype (a 

right valve) are figured in this paper. This right 

valve was in Tucker's possession at the time the 

original species was described (Tucker, 1934, p. 

615), the locality description for this specimen is 

more exact than that given for the other specimens, 

and, as the right valve is most distinctive of this 

species, this valve is designated the lectotype. Cotypes are highly artificial categories. According to 

present practice, the three specimens would have 

syntypic status. The selection of an unfigured "syntype" as lectotype was necessitated by the uncertainty of the depository of the originally figured 

specimen. This right valve which is designated the 

lectotype agrees quite well with the details from the 

figure of Tucker's original specimen. 

The left-valve cotype of Tncker (USNM 2498) 

was collected by E. Ruffin in 1863 from St. Marys 

River, Md. The right-valve paratype of Tucker 

(USNM 193448) was collected by Harris in 1891 

from the St. Marys River, Md., along the eastern 

bank for perhaps 0.5 mile (0.8 km) above the mouth 

of St. Inigos Creek. Many of the larger specimens 

collected were float on the beach (USGS 1oc. 2342). 

Chesapecten middlesexensis (Mansfield) 

Plate 5, figures 1, 2; plate 7, figures 4, 11 

1928. Pecten madisonius Say var. Mansfield, p. 10, pl. 2, 

fig. 1; pl. 3, fig. 1. 

1936. Pecten ( Chlamys) santamaria middlesexensis Mansfield, p. 187, pl. 22, figs. 5, 6. 

1938. Chlamys (Lyropecten) madisonius richardsi TuckerRowland, p. 14-15, pL 3, figs. 4, 5. 

Original description.-Pecten santamaria middlesexensis is like santamaria Tucker, ss., in its major 

features, but differs in the following minor features. 

Right valve slightly more inflated, the ears relatively wider, the byssal notch shallower, the ribs less 

rounded distally; left valve with less rounded ribs 

distally and coarser subradials on the ribs. The new 

subspecies is a descendant of P. santamaria. 

Diagnosis.-Chesapecten middlesexensis is a very 

large Chesapecten with a moderately deep byssal 

notch, valves of moderate thickness, small gapes between valves, and a highly variable number of ribs. 

Description.-Valves large, often greater than 120 

mm in height, suborbicular, biconvex, left valve 

strongly inflated, right valve moderately inflated, 

most adults longer than high, small gapes present 

along the ventral commissure owing to the inexact 

match of opposing valves, small gapes present opposing disk flanks, valves of moderate thickness. 

Auricles well developed, subequal, ctenolium obscured very early; anterior and posterior auricular 

denticles very reduced and in contact when valve 

closed, byssal notch shallow, byssal fasciole only 

slightly impressed. 

Plicae of right valve usually number 10 to 19, 

plicae elevated, broad, rounded, with flattened interspaces. Entire exterior with rows of fairly scabrous 

sculpture. 

Auricular denticles slightly expressed and in contact when valves together, resilial insertion deep, 

about two-thirds as long as high, with raised broad 

protuberances on either side of the insertion in the 

right valve. In the left valve the resilial insertion 

extends into the interior on a platform for about 

one-third of its length; in the right valve the platform is very reduced. Cardinal crura obsolete, pallial 

line and most muscle insertions only vaguely impressed, ribbing expressed over most of shell interior, adductor insertions moderately large. 

Illustrated specimens and measurements.-Lectotype, USNM 373074 right valve from Urbanna, Va. 

(USGS loc. 3915). Length 140.7 mm, height 129.5 
SYSTEMATIC DESCRIPTIONS 13 

mm, total length auricles along line of ligament 68.3 

mm, length of auricle anterior to center of resilial 

insertion 36.8 mm, angle formed by lines through 

origin of shel~l growth (at beak) and (both anteriorly and posteriorly) tangent to, but nowhere intersecting, edge of disk, 118°. Convexity of right valve 

19.5 mm (pl. 5, fig. 1; pl. 7, figs. 4, 11). 

Paratype, USNM 207693left valve from Urbanna, 

Va., riverfront between mouth of creek and '\Vharf 

of Weems line of Steamers (USGS loc. 3915). 

Length 154.1 mm, height 142.8 mm, total length 

auricles along line of ligament 7.6 mm, length of 

auricle anterior to center of resilial insertion 37.9 

mm, angle formed by lines through origin of shell 

growth (at beak) and (both anteriorly and posteriorly) tangent to, but nowhere intersecting, edge of 

disk, 122°. Convexity of left valve 32.0 mm (pl. 5, 

fig. 2) 0 

The following measurements (in millimeters) 

have been taken from the right valves of additional 

specimens of Chesapecten middlesexensis from map 

locality 42 at 13 ft (3.9 m) above beach level (see 

table 3 for explanation of the abbreviations) : 

Specimen LN HT LA LAA ANG ( 0 ) CON RIB 

1 162.7 155.2 84.1 42.5 117 30.4 12 2 127.8 126.5 68.9 36.6 110 22.2 12 3 121.4 113.6 58.4 28.9 111 19.1 13 4 110.3 104.9 54.1 32.6 117 17.2 19 5 103.9 100.1 46.8 29.2 111 14.8 16 6 103.5 98.2 46.4 28.8 114 14.7 18 7 101.6 101.8 46.9 27.2 110 16.1 18 8 94.4 88.2 47.8 24.9 110 15.7 10 9 79.2 77.2 40.8 24.4 107 12.0 22 10 71.7 73.8 38.0 21.5 114 10.3 13 11 42.5 45.0 23.1 15.4 96 8.4 18 12 39.7 42.3 24.6 15.8 95 4.9 16 13 24.9 27.6 16.4 10.7 91 3.5 19 

Occurrence.-The type locality is Urbanna, Middlesex County, Va. in the lower Virginia St. Marys 

Formation (of Mansfield, 1936) (fig. 1, loc. 16; 

USGS loc. 3915). 

Reference localities in the Virginia St. Marys 

Formation include the Nomini Cliffs (fig. 1, loc. 17; 

USGS loc. 25306), Hull Creek (fig. 1, loc. 18; USGS 

25307), Bowlers Wharf (fig. 1, loc. 19; USGS loc. 

25308) ; Urbanna Creek (fig. 1, loc. 20; USGS loc. 

25309), Freeport (fig. 1, loc. 21; USGS loc. 25310), 

Blands Wharf (fig. 1, loc. 22; USGS loc. 25311), 

White Oak Landing (fig. 1, loc. 14), Court House 

Landing (fig. 1, loc. 23; USGS loc. 25312), Corbin 

Pond (fig. 1, loc. 24; USGS loc. 25313), Gressitt 

Pond (fig. 1, loc. 25; USGS loc. 25314), White Landing (fig. 1, loc. 26; USGS loc. 25315), Romancoke 

(fig. 1, loc. 27; USGS loc. 25316), the lower greengray sandy clay bed from Chippokes Creek (fig. 1, 

loc. 28) to Cobham Bay (fig. 1, loc. 29; USGS loc. 

25318), the tan sandy shell bed (upper Virginia St. 

Marys Formation) from Sunken Marsh Creek (fig. 

1, loc. 30; USGS loc. 25319) to Cobham Bay, the 

Nottoway River at 631 bridge (fig. 1, loc. 31; USGS 

loc. 25320), Murfreesboro (lower foot of section) 

(fig. 1, loc. 32; USGS loc. 25321), and ~ miles (3.2 

km) above rte. 258 bridge at Murfreesboro (lower 

2 ft (0.5 m) of section) (fig. 1, loc. 33; USGS loc. 

25322). 

Comparison and remarks.-Chesapecten middlesexensis differs from C. jefjersonius in having more 

ribs, thinner valves, less inflated valves (especially 

the right valve) , less t; ght closure along the ventral 

commissure, a deeper byssal notch, a byssal fasciole 

which is differentiated from the auricle with respect 

to sculpture and elevation, and the exterior of the 

valves covered with moderately coarse scabrous 

lirae. 

Conrad (1862, p. 291) describes a pecten from the 

Miocene of Virginia and names it Pecten fraternus. 

The diagnosis of this specimen sounds very much 

like that of Chesapecten middlesexensis, but there is 

no figure, no locality other than Virginia, the type 

specimen appears to be lost, and the description is 

not complete enough to make a satisfactory judgement as to the intended species. Pecten tricarinatus 

Conrad (1867) may have been described from the 

same specimen Conrad used to describe P. fraternus 

Conrad (Heilprin, 1881). 

The type specimens of Chesapecten middlesexensis 

were collected by Frank Burns in 1903 from the 

river front at Urbanna, Middlesex County, Va., "between the mouth of Creek and Wharf of 'Veems line 

of Steamers on the Rappahannock River, Va." The 

right valve (USNM 373074) is here designated the 

lectotype of C. middlesea~ensis. 

The type specimens of Chlamys tnadisonius richardsi Tucker-Rowland (1938) are the specimens 

Mansfield (1928) illustrated as Pecten madisonius 

var. Although Mansfield (1936) thought these specimens represented a form occurring in the highest 

beds of the St. Marys Formation of Virginia, the 

specimens actually came from units which belong to 

the lower Virginia St. Marys Formation. 

Cbesapecten jeffersonius (Say) 

Plate 1; plate 2, figures 1-3; plate 5, figure1s 3-7; plate 7, 

figures 3, 10 

1824. Pecten Jeffersonius Say, p. 133, pl. 9, fig. 1. 

1840. Pecten Jeffersonius Say. Conrad, p. 46, pl. 22, fig. 1. 

1858. Pecten Jeffersonius Say. Emmons, p. 281, 282, fig. 199. 

1904. Not Pecten (Chlamys) jeffersonius Say. Glenn, p. 378, 

pl. C, fig. 2 ( = C. santa maria (Tucker) ) . 

1909. Pecten (Chlamys) jeffersonius Say. Grabau and 

Shimer, p. 502, fig. 673.f. 

1932. Chlamys (Lyropecten) jeffersonius Say. Mansfield, 

p. 59, pl. 11, fig. 1. 
14 MIOCENE AND PLIOCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

1936. Pecten (Chlamys) jeffersonius Say. Mansfield, p. 174, 

178, 179, 181, 184, 185. 

1938. Chlamys (Lyropecten) jeffersonius (Say). TuckerRowland, p. 19-20, pl. 1, figs. 3, 4; pl. 5, figs. 19, 20. 

1943. Chlamys (Lyropecten) jeffersonia (Say) Glenn. 

Gardner, p. 32-34, pl. 4, fig. 2. 

1959. Chlamys jeffersonia (Say). Mongin, p. 307-308, pl. 27, 

fig. 3. 

Original description and original comments.-

Shell rounded, convex, not quite equivalved, one of 

the valves being a little more convex than the other; 

the whole surface covered with approximate, scaly 

striae: ribs elevated, rounded, with six or seven 

striae on the back of each; intervening grooves profound: ears equal; sinus of the ear of the superior 

valve, not profound, being barely one eighth part of 

the length of the ear: within with broad rounded 

flattened ribs. 

Length five inches and three-tenths, breadth five 

inches and seven-tenths. 

Specimens of this truly fine shell are not uncommon. The Academy has been long in possession of 

several single valves, in an excellent state of preservation, obtained by my friend Mr. J. Gilliams, and 

others which were presented by Mr. w~ atson, who 

purchased them at the sale of the collection of the 

late Professor Barton. Mr. Finch has succeeded in 

obtaining entire specimens of the two valves of the 

same individual. I am of the opinion that Lister's 

plate 167, is intended to represent this shell, and 

that the singular appearance of the marginal striae 

in that figure is a deviation from the ordinary formation of the species, and is owing to the dislocation 

of the lines of increment, and obliteration of the 

longitudinal striae. Lister describes his specimens 

to be of a "blue-clay colour," in this respect perfectly corresponding with two specimens before me. 

On one of the specimens is an imperfect Astrae. 

Diagnosis.-Chesapecten jetfe?·sonius has a very 

small byssal notch in adult forms, a moderately 

heavy shell, a byssal fasciole hardly differentiated 

from the auricle with respect to sculpture and elevation, and a right valve which becomes quite convex, especially during later growth, whereas convexity in the left valve is seen during earlier growth. 

Description.-Valves large, often greater than 120 

mm in height, suborbicular, biconvex, left valve 

strongly inflated and right valve slightly less so, 

most adults longer than high, closure along ventral 

commissure tight with no gape, small gapes are present between opposing disk flanks, valves thick. 

Auricles well developed, subequal, ctenolium obscured very early, byssal notch extremely shallow, 

byssal fasciole broad but hardly differentiated from 

auricle with respect to sculpture and elevation. 

Plicae of right valve usually number between 9 

and 12, plicae elevated, broad, slightly rounded with 

interspaces. Entire exterior covered with reduced 

scabrous lirae which give an almost beaded appearance to the shell exterior. 

Anterior and posterior auricular denticles very 

reduced and in contact when valves closed, resilial 

insertions very deep, almost twice as high as long, 

and usually directed very slightly posteroventrally; 

resilial insertion extends into interior on a platform 

for about one-third of its length in the left valve; in 

the right valve the platform is hardly extended; 

cardinal crura obsolete, pallial line very impressed, 

impressed narrow insertions of circular pallial muscles, interior ribbing expressed chiefly in extrapallial 

margin area of shell and in adductor muscle region, 

adductor insertions large with a distinct nonstriate 

portion slightly over half the size of the striate portion, dorsal margin does not encroach on ligament, 

gill suspensor insertions impressed, trace of outer 

ligament broad. 

Ill-ustrated specimens and measurements.-Lectotype, British Museum (No. L13212), paired valves 

probably from Virginia. Length 99.2 mm, height 

93.1 mm, total length auricles along line of ligament 

50.1 mm, length of auricle anterior to center of 

resilial insertion 24.2 mm, angle formed by lines 

through origin of sheH growth (at beak) and (both 

anteriorly and posteriorly) tangent to, but nowhere 

intersecting, edge of disk, 109°. Convexity of paired 

specimen 42.3 mm (pl. 5, figs. 5-7). 

USNM 193445, paired valves from Cobham 

Wharf, right bank of James River, Va., 18 to 22 

ft (6 m) above beach, Zone 1 of Mansfield, Yorktown Formation (USGS loc. 25331). Length 135.0 

mm, height 144.1 mm, total length auricles along 

line of ligament 80.6 mm, length of auricle anterior 

to center of resilial insertion 36.6 mm, angle formed 

by lines through origin of shell growth (at beak) 

and (both anteriorly and posteriorly) tangent to, but 

nowhere intersecting, edge of disk, 117°. Convexity 

of left valve 36.6 mm. Convexity of right valve 31.2 

mm (pl. 2, figs. 1-3; pl. 5, figs. 3, 4; pl. 7, figs. 3, 10). 

The following measurements (in millimeters) 

have been taken from the right valves of additional 

specimens of Chesapecten jeffersonius from map 

locality 42 at 20 ft (6 m) above beach level (see 

table 3 for explanation of the abbreviations) : 
SYSTEMATIC DESCRIPTIONS 15 

Specimen LN HT LA LAA ANG ( 0 ) CON RIB 

1 137.7 124.6 64.6 31.6 123 26.5 10 

2 133.4 120.9 74.3 37.4 122 29.9 9 

3 120.8 110.6 65.8 33.6 116 24.0 8 

4 120.2 107.6 61.5 30.9 121 25.1 9 

5 107.6 96.7 53.1 28.5 117 18.2 9 

6 65.6 59.9 31.0 16.9 113 11.4 10 

7 40.9 40.5 25.0 13.3 103 69 10 

8 36.9 38.7 19.2 9.1 99 7.3 10 

9 31.2 31.4 19.8 10.1 102 5.7 10 

10 30.1 30.0 20.0 10.1 99 4.8 9 

Occurrence.-The type locality is Virginia, probably from the James River near City Point or the 

York River at Yorktown. Additional Virginia localities (Yorktown Formation Zone 1 of Mansfield) include 2 miles (3.2 km) below Grays Point Bridge on 

the Rappahannock River (fig. 1, loc. 34; USGS loc. 

25323), Glebe Neck (beach to +3 ft (1 m)) (fig. 1, 

loc. 35; USGS loc. 25324), Mouth of Felgate's Creek 

(at beach level) (fig. 1, loc. 36; USGS loc. 25325), 

Zone 1 Yorktown Formation (of Mansfield) from 

Chippokes Creek to Grove Wharf (beach to +5 ft 

(1.5 m)) (fig. 1, loc. 37; USGS loc. 25326), and 

Delaware at rte. 671 bridge over Nottoway (lower 

1.5 ft (0.5 m)) (fig. 1, loc. 38; USGS loc. 25327). 

Comparison and remarks.-Chesapecten jeffersonius differs from C. septenarius in having more 

plicae (usually 9 to 12 as opposed to approximately 

6 or 7) , which are less broadened, less flattened on 

the summits, and less square sided, with the exterior 

covered with less coarse scabrous sculpture. Chesapecten jeffersonius differs from C. tnadisonius in 

having more convex valves, in having fewer and 

more elevated plicae; also, in the exterior sculpture 

being less "beaded" in appearance, and in the interior ribbing being expressed in the adductor muscle 

region. 

The following information (the title and the abstracted text) was given by Huddesford (1770) in 

a new edition of Lister's publications. 

The following NoTES ~nd O~:sERVATroNs were written by Dr. L~fler in a 

Copy of this Work ( confifiing chieRy of Proof Plates) now in the 

Poilcflion of Mr. lr.gham FQrjler, Merchant in London, by whom it 

\Vas mofl: obligingly communicated to the Editor. A. D. 1769. 

Tab. !'\•. 

167. 4• This is the bigge!l: Scallop I have feen; it is alto thick and heavy; it was of a bue clay 

Colour, but it was apparently old and weather beaten, fo that the natural Beauty of the living 

Fifh was perilhed. It is but fhallow. It has but ten Ridges; but the Furrows be"twixt them 

are very deep, and cut down almo!l: at right Angles ; at the very Head of the Shell, betwixt 

the Eare~, the Furrows are very deep, though fmall, and end all in a Point vt:ry elegant; 

whereas in the two former Shells the Heads are fmooth, and the Ridges and Furrows in that 

Part are almo!l: obliterated. The Ridges and Furrows here are all of them curiouOy 11reaked . 

.r'\ gooll Inch from the Bottom of the Sheli the Hidges and Furrows become fuJdenly of another Fa01ion and Grain, and are, as it were, tranfverlly waved am! fmooth. 

I never faw but two of thefe, and they were both bigger or upper Shd!s of the Scallop. 

N.B. That is a Fault in the Engraver to have made the Hidgc~ tuwJrJs the Head a little 

crooked, whereas they lie all very ttrait and run to a Point. 

A translation of Lister's original Latin description (Lister, 1687, p. 69) is as follows: By far the 

largest of all scallops presented with 8 or 10 ridges 

at the highest point and similarly deep grooves. 

It should be noted that there was no separate text 

for each figure, but that the entire description was 

included with each figure (as reproduced in pl. 1). 

A probable syntype of C. jeffersonius was obtained from the British Museum, the repository of 

of the fossil material which J. Finch loaned Say to 

describe. This specimen agrees with Say's figure in 

size, although the details of sculpture are apparently 

artistically generalized possibly from several specimens (synthetograph). This articulated specimen 

from the British Museum is here designated as the 

lectotype. 

Chesapecten septenarius (Say) 

Plate 6, figures 5-7; plate 7, figures 2, 9 

1824. Pecten septenarius Say, p. 136, pl. 9, fig. 3. 

1840. Pecten septenarius Say. Conrad, p. 4 7, pl. 22, fig. 2. 

1856. Pecten septenarius Say. Tuomey and Holmes, p. 31, 

pl. 13, figs. 1-4. 

1898. Pec.ten Jeffersonius var. septenarius Say. Dall, p. 722. 

1904. Pecten ( Chlamys) jefferson ius var. septenarius Say. 

Glenn, p. 379, pl. C, fig. 4. 

1936. Pecten ( Chlamys) je_fje1·sonius septenarius Say. Mansfield, p. 174, 179, 184, 185. 

1936. Pecten ( Chlarnys) jeffersonius palmyrensis Mansfield, 

p. 187-188, pl. 23, fig. 3. 

1938. Chlarnys (Lyropecten) jeffersonius septenarius (Say). 

Tucker-Rowland, p. 20-21, pl. 3, fig. 15. 

1938. Chlamys (Lyropecten) jeffersonius palrnyrensis 

(Mansfield). Tucker-Rowland, p. 20, pl. 2, fig. 11. 

1964. Pecten jeffersonius septenarius Say. Darby and Hoyt, 

pl. 18, fig. 16. 
16 MIOCENE AND PLIOCENE CHESAPECTEN OF EAS.TERN NORTH AMERICA 

Original description.-Shell convex, suborbicular: 

auricles subequal: surface with numerous slightly 

scaly striae, and about seven remote ribs, of which 

the three intermediate ones are much elevated, 

rounded, or slightly flattened on the top. Length 

nearly two inches and seven-tenths. 

Diagnosis.-Chesapecten septenarius has very 

heavy deposits of calcite on the interior of the 

valves, usually 6 to 7 plicae, a shallow byssal notch, 

an only slightly differentiated byssal fasciole, and 

broad ribs with flattened summits and square sides. 

Description.-Valves moderate in size (adults 

about 110 mm in height), suborbicular, biconvex, 

left valve quite inflated and right valve slightly less 

inflated, most adults slightly longer than high, 

valves close tight'ly along ventral margin, small 

gapes between opposing disk flanks, shell thick and 

heavy. 

Auricles well developed and subequal, byssal notch 

very small, ctenolium obscured very early in growth, 

auricular denticles very reduced and in contact with 

opposing denticles, byssal fasciole inconspicuous and 

little differentiated with respect to auricle and sculpture on auricle. 

Plicae usually number about 6 to 7, plicae broad, 

flattened, square sided, with very slightly curved 

interspaces, entire exterior covered by moderately 

coarse scabrous sculpture. 

Trace of outer ligament broad, dorsal margin not 

encroaching on ligament, resilial insertion deep and 

about half as long as high, in left valve resilial in- 1 

sertion extends to the interior for about one -third of 

its height and in the right valve there is no extension. In the right valve, moderate prominances apparent on either side of the resilial insertion. Cardinal crura obsolete. 

PalHal line and muscle insertions moderately expressed. Adductor insertions very large. Ribbing in 

interior expressed largely along extrapallial margin 

but also in pallial area despite heavy deposits of 

calcite. 

Illustrated specimens and measurement.~.-Holotype (monotype), British Museum (No. L13197). 

Left valve from Virginia (probable locality). 

Length 64.9 mm, height 67.3 mm, total length auricles along line of ligament 40.1 mm, length of auricle 

anterior to center of resilial insertion 22.3 mm, 

angle formed by lines through origin of shell growth 

(at beak) and (both anteriorly and posteriorly) tangent to, but nowhere inters~cting, edge of disk, 109°. 

Convexity of left valve 16.9 mm (pl. 6, figs. 5, 6). 

USNM 193446 paired valves from Lee Creek mine 

on right bank of Pamlico River, N.C. (USGS loc. 

25334). Length 116.5 mm, height 109.2 mm, total 

length auricles along line of ligament 60.6 mm, 

length of auricle anterior to center of resilial insertion 31.0 mm, angle formed by lines through origin 

of shell growth (at beak) and (both anteriorly and 

posteriorly) tangent to, but nowhere intersecting, 

edge of disk, 108°. Convexity of left valve 30.0 mm. 

Convexity of right valve 24.1 mm (pl. 6, fig. 7; pl. 7, 

figs. 2, 9). 

The fol:lowing measurements (in millimeters) have 

been taken from the right valves of additional specimens of Chesapecten septenarius from map locality 

45 (USGS loc. 25334) (see table 3 for explanation 

of the abbreviations) : 

Specimen LN HT LA LAA ANG ( 0 ) CON RIB 

1 129.2 122.7 (1) (1) 110 21.4 6 

2 92.9 87.4 50.2 26.5 114 17.7 9 

3 88.3 85.3 53.9 27.4 103 18.8 6 

4 77.8 73.2 48.8 24.6 102 14.3 5 

5 70.9 !l8.2 42.0 21.4 107 14.4 6 

6 42.1 42.9 24.7 12.8 105 7.6 7 

1 Broken. 

Occurrence.-The type locality is Virginia, probably from the James River near City Point or the 

York River at Yorktown. 

Additional Virginia localities (Yorktown Formation Zone 2 of Mansfield) include Petersburg (first 

6 ft (1.8 m) above Virginia St. Marys Formation 

contact with Zone 2 Yorktown) (map loc. 39; USGS 

loc. 25328), 2 miles (3.2 km) above Yorktown (fig. 

1, loc. 40; USGS loc. 25329), Rices Borrow Pit (fig. 

1, loc. 41; USGS loc. 25330), Cobham Wharf (top 

shell bed) (fig. 1, loc. 42; USGS loc. 25336), Kings 

Mill Wharf (fig. 1, loc. 43; USGS loc. 25332), Lee 

Creek mine pit (fig. 1, loc. 45; USGS loc. 25334). 

Comments.-The specimen figured here (pl. 6, 

figs. 5, 6) was obtained from the British Museum. It 

agrees almost exactly with Say's figured specimen, 

including the large breaks in the shell margin. The 

specimen is labeled "N. Amer." Say apparently had 

only one specimen. _ 

Cbesapecten madisonius (Say) 

Plate 6, figures 1-4; plate 7, figures 1, 7, 8 

1824. Pecten Madisonius Say, p. 134. 

1862. Pecten Edgecombensis Conrad, p. 291. 

1863. Pecten edgecombensis Conrad. Conrad, p. 581. 

1863. Not Pecten Madisonius Say. Con.rad, p. 581, ( = C. 

nefrens). 

1894. Not Pecten Madisonius Say. p. 30, pl. 4, figs. 1-5. 

1904. Pecten jeffersonius var. edgecmnbensis (Conrad). 

Glenn, p. 379, pl. C, fig. 3 [locality not St. Marys 

River]. 

1904. Nort Pecten (Chlamys) madisonius Say. Glenn, p. 377, 

pl. C, fig. 1, (=C. nefrens). 

1909. Nort Pecten (Chla;mys) madisonius Say. Grabau and 

Shimer, p. 502, fig. 673c, (=C. nefrens). 
SYSTEMATIC DESCRIPTIONS 17 

1936. Not Pecten (Chlamys) madisonius Say. Mansfield, p. 

174, 176, 177, 184, (=C. nefrens). 

1938. Not Chlamys (Lyropecten) madisonius (Say). TuckerRowland, p. 9-11, pl. 1, fig. 1, 2 (=C. nefrens). 

1938. Chlamys (Lyropecten) jeffersonius edgecombensis 

(Conrad). Tucker-Rowland, p. 15-16, pl. 2, fig. 5; 

pl. 4, fig. 6. 

1941. Not Chlamys (Lyropecten) madisonius (Say). Schoonover, p. 28-37, pl. 2, figs. 3, 5-6; pl. 3, figs. 1-3, 6; 

pl. 4, figs. 1-4; pl. 5, figs. 1-3. 

1943. Not Chlamys (Lyropecten) madisonia (Say) Glenn. 

Gardner, p. 32, pl. 4, fig. 5; pl. 9, fig. 7, (=C. 

nefrens). 

1959. Not Chlamys madisonia (Say). Mangin, p. 309-314, 

pl. 26, figs. 1a, b (=C. nefrens). 

Complete original description.-Much compressed, 

with about sixteen striated ribs. 

Shell rounded, much compressed; the whole surface covered with scaly striae: ribs elevated, 

rounded, with about three striae on the back of each; 

intervening grooves rather profound: ears equal, 

sinus of the ear of the superior valve profound, 

extending at least one third of the length of the ear. 

Length rather more than four inches and a half; 

breadth four inches and four-fifths. 

In magnitude this shell is justly entitled to compare with the preceding; but it differs in being much 

less convex, and in having a much more profound 

sinus in the ear of the superior valve. Three specimens, from which the above description was taken, 

belong to the Academy and were presented by Mr. 

Watson. 

Description.-Valves moderately large (often 

greater than 100 mm in height), suborbicular, biconvex, left valve strongly inflated and right valve 

only slightly less so, most adults longer than high, 

closure along ventral commissure tight with no gape, 

small gapes are present between opposing disk 

flanks, valves thick. 

Auricles very well developed, subequal, ctenolium 

obscured very early, byssal notch extremely shallow, 

byssal fasciole broad but hardly differentiated from 

auricle with respect to sculpture and elevation. 

Plicae of right valve usually number 14 to 16, 

plicae not greatly elevated, but broad, rounded, with 

rounded interspaces. Entire exterior covered with 

rows of closely spaced reduced scabrous lirae. 

Trace of outer ligament broad, resilial insertion 

moderately deep and two-thirds as long as high, in 

left valve resilial insertion extends into shell on 

platform for about one-third of its height and in 

right valve for about one-fourth of its height. Posterior auricular denticles weak and in contact when 

valves closed; anterior auricular denticles moderately expressed. Cardinal crura obsolete, pallial line 

impressed, narrow insertions of pallial muscles 

fairly impressed, interior ribbing expressed almost 

exclusively in extrapallial margin of shell. Adductor 

insertions large with a distinct nonstriate portion 

sUghtly over half the size of the striate portion, dorsal margin does not encroach upon ligament, gill 

suspensor insertions impressed. 

Illustrated specirnens and ·measurements.-Lectotype, Academy Nat ural Sciences of Philadelphia 

(ANSP 31787) left valve from Virginia (probable 

locality). Length 123.9 mm, height 116.1 mm, total 

length auricles along line of ligament 62.5 mm, 

length of auricle anterior to center of resilial insertion 31.7 mm, angle formed by lines through origin 

of shell growth (at beak) and (both anteriorly and 

posteriorly) tangent to, but nowhere intersecting, 

edge of disk, 118°. Convexity of left valve 29.9 mm 

(pl. 6, figs. 3, 4). 

USNM 193447 right valve from Cobham Wharf, 

right bank of James River, Surry County, Va. +24.5 

to +28 ft (8 m) Zone 2 Yorktown Formation (of 

Mansfield) (USGS loc. 25336). Length 112.8 mm, 

height 103.7 mm, total length auricles along line of 

ligament 59.7 mm, length of auricle anterior to 

center of resilial insertion 29.0 mm, angle formed by 

lines through origin of shell growth (at beak) and 

(both anteriorly and posteriorly) tangent to, but 

nowhere intersecting, edge of disk, 115°. Convexity 

of right valve 19.0 mm (pl. 6, figs. 1, 2; pl. 7, figs. 

1, 8). 

The following measurements (in millimeters) 

have been taken from the right valves of additional 

specimens of Chesapecten ·ma,disonius from map locality 41 (USGS loc. 25330) (see table 3 for explanation of the abbreviations) : 

Specimen LN HT LA LAA ANG ( 0 ) CON RIB 

1 148.2 142.7 90.6 46.7 114 29.9 16 

2 142.0 131.3 81.8 42.0 117 25.1 14 

3 124.1 113.4 74.5 40.8 111 20.3 15 4 116.9 109.5 67.7 35.9 111 16.5 14 

5 109.8 105.6 63.4 34.5 112 19.7 16 f) 96.5 91.2 60.2 31.3 111 17.0 13 7 91.1 88.7 57.7 30.6 107 12.1 16 8 80.3 79.4 46.8 25.1 103 10.2 16 9 77.7 73.6 43.6 22.8 112 10.3 15 lO 74.7 72.6 46.8 24.6 106 11.8 14 11 63.6 63.1 37.7 19.6 105 9.8 16 12 55.8 56.8 32.8 18.1 103 6.2 14 13 46.3 46.3 26.8 14.4 102 7.5 14 14 43.6 44.7 27.1 14.9 100 7.2 16 

Occurrence.-The type locality is unknown but 

probably Virginia or North Carolina. 

Additional Virginia localities (Yorktown Formation Zone 2 of Mansfield) include the York River 

from Yorktown (fig. 1, loc. 40 ; USGS loc. 25329) 

to 2 miles (3.2 km) below Yorktown (fig. 1, loc. 46; 

USGS loc. 25335), Cobham Wharf (fig. 1, loc. 42; 
18 MIOCENE AND PLIOCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

USGS loc. 25336), Rices Pit (fig. 1, loc. 41; USGS 

loc. 25330), and Sycamore Bend (fig. 1, loc. 44; 

USGS loc. 25333) . 

North Carolina localities (Yorktown Formation 

Zone 2 of Mansfield) include the rte. 258 bridge at 

Murfreesboro (6-9 ft (2+ m) above the Virginia 

St. Marys contact and Zone 2 Yorktown) (fig. 1, 

loc. 33; USGS loc. 25322), and Lee Creek Pit (fig. 1, 

loc. 45 ; USGS loc. 25334) . 

Comments.-The reasons for restricting the name 

Chesapecten madisonius to specimens found in Zone 

2 of the Yorktown Formation are discussed in the 

section on C. nefrens. In Say's description of C. 

madisonius, he was probably considering as the superior valve the right valve, which in this genus, 

unlike many other Pectinidae, is the lower valve. 

The right valve does have a distinct sinus which is 

especially pronounced in younger individuals. Certainly, as Say was comparing the holotype of C. 

jefjersonius', the specimens of Pecten clintonius, and 

the holotype (monotype) of C. septenarius with the 

right valves of C. madisonius, he would have seen a 

pronounced sinus, which is about one-third the 

length of the ear in smaller individuals (pl. 7, fig. 

7). Although the three "striae" are usually more 

prominent in specimens of C. nefrens, the number 

of striae in the specimens we consider to be C. tnadisonius from the Yorktown Formation is usually 

three, particularly in specimens from certain beds. 

Say indicates in his description that the number of 

striae was not constant in the specimens before him. 

Such inconstancy would be normal for C. rnadisonius 

but unusual for a collection of specimens of C. 

nefrens. Also the term "striae" is hardly applicable 

to the scaly sculpture common in C. nefrens, whereas it is quite applicable to C. madisonius and particularly to the specimen here illustrated as the lectotype of C. madisonius (pl. 6, fig. 3, 4). In number 

of ribs both C. nefrens and C. madison·ius are similar. 

Prior to this paper and before recognition of Say's 

specimen at the Philadelphia Academy, a worker at 

the British Museum had selected from Finch's collection a specimen which he considered making a 

lectotype of C. madisonius. The specimen he selected is a Yorktown Formation C. madisonius. This 

selection would indicate that the material in Finch's 

collection which most resembles the description of 

C. madisonius is a Virginia Yorktown specimen. Say 

states that C. madisonius has about 16 ribs. Our lectotype has 15 ribs, while the British Museum speciment (a right valve) has 16 ribs. 

Conrad applied Say's name 1nadisonius to the 

Maryland species and consequently gave another 

species name, edgecombensis, to what was actually 

the P. madisonius of Say. All known references to 

C. madisonius in the Calvert and Choptank Formations are to the species here called nej1·ens and all 

known references to C. madisonius in the Yorktown 

Formation are misidentifications of the species formerly known as Pecten edgecotnbensis, here shown 

to be the madisonius of Say (1824). 

Dall (1898) incorrectly recorded the species (Pecten edgecombensis=C. rnadisonius) from Langley's 

Bluff, Md. Glenn (1904) incorrectly recorded this 

Yorktown species (P. edgeco·mbensis) from the St. 

Marys River, Md., and also from Langley's Bluff, 

Md. Tucker-Rowland (1938) incorrectly recorded 

this species (P. edgecotnbens7s) from Little Cove 

Point, Md.. and her other "Maryland" locality 

(Grove Wharf) is in Virginia. There are no known 

occurrences of Chesapecten edgecombensis (=C. 

1nadisonius) in Maryland. Glenn's (1904) figured 

specimen (pl. C, fig. 3) of this species (P. edgecombensis) is wrongly labeled "St. Marys River" in the 

plate explanation. The specimen actually came from 

Edgecombe County, N.C., and was collected by J. L. 

Bridges and entered in the National Museum catalogue in 1863 (type locality and collection from 

which original specimens of edgeco·mbensis came). 

Plate 6, figures 3 and 4 show the specimen which 

appears to be the only one remaining from the three 

original type specimens of Say's Pecten 1nadisonius. 

It agrees with Say's original description in having 

three striae on the back of elevated ribs, in having 

equal ears, in the length and height measurements, 

and in being much more compressed than C. je.ffersonius. This specimen was obtained from the Philadelphia Academy of Natural Sciences. 

Collections in the British Museum apparently lack 

the type specimens. Because the specimens of C. 

madisonius were not collected by Finch, they probably were not sent to England. Some small specimens of C. madisonius are found in the Finch collection. 

It should be noted that the specimen TuckerRowland (1938) figures (pl. 4, fig. 8) and mentions 

as the possible holotype of C. ,madisonius (Say, 

1824) is actually a specimen collected in 1887 by 

Frank Burns from New Jersey (according to the 

U.S. Geological Survey Cenozoic Collection Catalogue). 

LOCALITY REGISTER 

Locality USGS No. in locality Description 

.fignre 1 No. 

1 25293 Randall Cliff, west shore of Chesapeake 

Bay, 0.5 mile (0.8 km) south of Chesa-
Locality 

No. in figure 1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

USGS 

locality 

No. 

25294 

25295 

25296 

25297 

25298 

25299 

25300 

25306 

25301 

25302 

25303 

25304 

25058 

25059 

LOCALITY REGISTER 

Description 

peake Beach, Calvert County, Md. (North 

Beach 7%-minute quad.), in Zone 10 at 

approximately 35ft (10.6 m) above beach; 

Calvert Formation. 

Plum Point, west shore of Chesapeake Bay, 

Calvert County, Md. (Prince Frederick 

7 1h-minute quad.), in Zone 10 at beach 

level; Calvert Formation. 

Camp Roo·sevelt, 0.5 mile below, on west 

shore of Chesapeake Bay, Calvert County, 

Md., (North Beach 7%-minute quad.) in 

Zone 10 at 15 ft ( 5 m) above bea~h; 

Calvert Formation. 

Governor Run, west shore of Chesapeake Bay, 

Calvert County, Md., (Prince Frederick 

7%-minute quad.), in Zone 14 at beach 

level; Calvert Formation. 

Scientist's Cliffs, west shore of Chesapeake 

Bay, Calvert County, Md., (Prince Frederick 7%-minute quad.), in Zone 16 at 

30 ft (9 m) above beach; Choptank 

Formation. 

Calvert Beach, west shore of Chesapeake 

Bay, Calvert County, Md. (Cove Point 

7%-minute quad.), in Zone 16 at beach 

level to 5 ft (1.5 m) Choptank Forma,tion. 

Camp Conoy, west shore of Chesapeake Bay, 

Calvert County, Md., (Cove Point 7%-

minut.e quad.), in Zone 19 at 8ft (2m) 

above beach; Choptank Formation. 

Cove Point, west shore of Chesapeake Bay, 

Calvert County, Md., (Cove Point 7%-

minute quad.) ; St. Marys Formation 

(USGS 25300) and Zone 19 Choptank 

Formation (USGS 25306). 

Flag Pond, west shore of Chesapeake Bay, 

Calvert County, Md., (Cove Point 7%-

minute quad.) ; Ghoptank Formation, Zone 

19. 

Little Cove Point, west shore of Chesapeake 

Bay, Calvert County, Md., (Solomons Island 7%-minute quad.), in upper shell bed 

at +11 ft (3+ m); St. Marys Formation. 

Langley's Bluff, west shore of Chesapeake 

Bay, 2 miles (3.2 km) southeast of Hermanville, St. Marys County, Md. (St. 

Marys City 7%-minute quad.), shell bed at 

beach level; St. Marys Formation. 

Windmill Point, right bank of St. Marys 

River, opposite mouth of St. Inigoes Creek, 

St. Marys County, Md. (St. Marys City 

7%-minute quad.), tan shell bed at 3 ft 

(1 m) elevation; St. Marys Formation. 

Essex Mill, 3.5 miles (5.6 km) south of Tappahannock on Mill Creek, a tributary of 

Piscataway Creek, Essex County, Va., 

(Dunnsville 71;2-minute quad.), in mill 

race; St. Marys Formation. 

White Oak Landing, 2.5 miles ( 4.0 km) east 

of King William Court House, on right 

bank of Mattaponi River, just upriver of 

Locality 

No. in 

fiqure 1 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

USGS 

locality 

No. 

25061 

3915 

25306 

25307 

25308 

25309 

25310 

25311 

25312 

25313 

25314 

25315 

19 

Description 

old wharf piling, very shelly dark clay, 

from beach to +1 ft (0.3 m) (King and 

Queen 7%-minute quad.); St. Marys Formation. 

Mantua Landing, 2.5 miles ( 4.0 km) east of 

King William Court House, on right bank 

of Mattaponi River, very shelly dark green 

gray clay (1 ft (0.3 m) thick) resting on 

unconformity located 1 ft (0.3 m) above 

beach level (King and Queen 7%-minute 

quad.) ; St. Marys Formation. 

Urbanna, riverfront between mouth of creek 

and Wharf of Weems line of steamers, 

right bank of Rappahannock River, Middlesex County, Va. (Urbanna 7%-minute 

quad.); upper Virginia St. Marys Formation. 

Nomini Cliffs, right bank of Potomac River, 

Westmoreland County, Va. (Stratford 7%-

minut,e quad.), 35 ft (10 m) elevation); 

upper Virginia St. Marys Formation. 

Hull Creek, right bank of Potomac River, 

Northumberland County, Va. (Heathsville, 

7%-minute quad.); upper Virginia St. 

Marys Formation. 

Bowler's Wharf, 1-2 miles (2.4 km) south 

of Bowler's Wharf right bank of Rappahannock River, Essex County, Va. (Morattico 7%-minute quad.), Spisula bed from 

beach level to +12 ft ( +3 m), upper Virginia St. Marys Formation. 

Urbanna Creek, 0.5 mile south of Urbanna, 

Middlesex County, Va. (Urbanna 7%-

minute quad.), from beach to 3 ft (1 m); 

upper Virginia St. Marys Formation. 

Freeport, just above mouth of Harper Creek, 

right bank of Piankatank River, Gloucester 

County, Va. (Saluda 7%-minute quad.), 

at +6 ft (2 m) in shell bed; upper Virginia St. Marys Formation. 

Blands Wharf, just below mouth of Harper 

Creek, right bank of Piankatank River, 

Gloucester County, Va., (Wilton 7%-

minute quad.), at +7 ft (2 m) in shell 

bed; upper Virginia St. Marys Formation. 

Courthouse Landing, left bank of Mattaponi River, King and Queen County, Va. 

(King and Queen 7%-minute quad.); from 

beach level to +4 ft ( +1 m), upper Virginia St. Marys Formation. 

Corbin Pond, where rte. 14 crosses Corbin 

Creek, King and Queen County, Va. (West 

Point 71h-minute quad.) ; in streamcut 

below bridge; upper Virginia St. Marys 

Formation. 

Gressitt Mill, in millrace, King and Queen 

County, Va., (Gressitt 7~i:J-minute quad.); 

upper Virginia St. Marys Formation. 

0.5-1.5 miles (0.8-2.4 km) below White 

Landing, left bank of Pamunkey River, 
20 

Locality 

No. in 

figure 1 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 

MIOCENE AND PLIOCENE CHESAPECTEN OF EASTERN NORTH AMERICA 

USGS 

locality 

No. 

Description 

King William County, Va. (New Kent 

7%-minute quad.); series of small exposures as much as 4 ft ( 1 m) above water; 

lower Virginia St. Marys Formation. 

25316 1.1 miles (1.76 km) northwest of Romancoke, 

left bank of Pamunkey River, King William County, Va. (West Point 7'h-minute 

quad.), at water level; upper Virginia St. 

Marys Formation. 

25317 Mouth of upper Ghippokes Creek at Claremont, right bank of James River, Surry 

County, Va. (Claremont 7 'h-minute 

quad.), at water level to +8 ft ( +2 m); 

lower Virginia St. Marys Formation. 

25318 Bluffs at Ghippokes Plantation, nght bank 

of James River at Cobham Bay, Surry 

County, Va. (Hog Island 7'h-minute 

quad.), at beach level; lower Virginia St. 

Marys Formation. 

25319 Just below mouth of Sunken Marsh Creek, 

right bank of J arne's River (Claremont 

7'h-minute quad.), at +30 to +32 ft ( +9 

m) ; upper Virginia St. Marys Formation. 

25320 Just below Rte. 6·31 bridge over Nottoway 

River right bank, Sussex County, Va. (Sebrell 71hminute quad.), lower 3 ft (1 m); 

upper Virginia St. Marys Formation. 

25321 0.5 mile (0.8 km) above the rte. 258 bridge 

at Murfreesboro, Meherrin River, Hertford County, N.C. (unmapped quad.), at 

+1 ft (0.3 m); upper Virginia St. Marys 

Formation. 

25322 2 miles (3.2 km) above the rte. 258 bridge 

at Murfreesboro, right hank of Meherrin 

River, Hertford County, N.C. (unmapped 

quad.), from water level to +3 ft ( +1 

m); upper Virginia St. Marys Formation. 

25323 2 miles ( 3.2 km) below Greys Point Bridge, 

right bank of Rappahannock River, Middlesex, County, Va. (Deltaville 7'h-minute 

quad.) ; Zone 1 of Yorktown Formation. 

25324 Glebe Neck, left bank of Piankatank River, 

Middle•sex County, Va. (Wilton 7'h-minute 

quad.), from beach level to +3 ft ( +1 

m); Zone 1 of Yorktown Formation. 

25325 Mouth of Felgates Creek, right bank of 

York River, York County, Va. (Clay Bank 

7'h-minute quad.), at beach level; Zone 1 

of Yorktown Formation. 

25326 Grove Wharf, left bank of James River 

James City County, Va. (Hog Island 71h~ minute quad.), beach level to +5 ft (1.5 

m) ; Zone 1 of Yorktown Formation. 

25327 0.2 mile (0.32 km) above rte. 671 bridge at 

Delaware, left bank of Nottoway River, 

Southamp·ton County, Va., (Courtland 7'hminute quad.)~ water level to +1.5 ft (0.5 

m); Zone 1 of Yorktown Formation. 

25328 Lieutenant Run, 0.2 mile (0.32 km) upstream of Baylor Lane, Petersburg, Va. 

(Petersburg 7'h-minute quad.) first 6 ft 

Locality 

No. in 

figure 1 

40 

41 

42 

43 

44 

45 

46 

USGS 

locality 

No. 

25329 

25330 

25331 

25336 

25332 

25333 

25334 

25335 

Description 

(2 m) above Virginia St. Mary's Formation contact with Zone 2 Yorktown; Zone 

2 of Yorktown Formation. 

2.0 miles (3.2 km) above Yorktown, right 

bank of York River, York County, Va. 

(Clay Bank 7%-minute quad.), float on 

beach; Zone 2 of Yorktown Formation. 

Rice's borrow pit at intersection of Fox Hill 

Road and Harris Greek Road, Hampton, 

Va. (Hampton 71h-minute quad.); Zone 2 

of Yorktown Formation. 

Cobham Wharf, right bank of James River, 

Surry County, Va. (Surry 7'h-minute 

quad.) ; Zone 1 Yorktown Formation 18 

to 22 ft (about 6 m) above beach and 

Zone 2 Yorktown Formation 24.5 to 

28 ft (about 8 m) above beach; Boundary between upper and lower Virginia St. 

Marys Formation is at 5 ft ( 1.5 m) 

above beach. 

0.6 mile (0.96 km) below old Kings Mill 

Wharf Pilings, left bank of James River, 

James City County, Va. (Hog Island 7'hminute quad.); Zone 2 of Yorktown Formation. 

Sycamore Bend, 1.25 mile·s (2 km) southeast 

of Delaware, left bank of Nottoway River, 

Southampton County, (Franklin 7'hminute quad.); Zone 2 of Yorktown Formation. 

Lee Creek mine (Texas Gulf Sulfur phosphate mine) north of Aurora, N.C., on the 

right bank of the Pamlico River (Aurora 

7%-minute quad.); Zone 2 of Yorktown 

Formation. 

2.0 miles (3.2 km) below Yorktown at the 

Moore House, right hank of York River, 

York County, Va., (Poquoson West 7'hminute quad.); Zone 2 of Yorktown Formation. 

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Brann, D. C., and Kent, L. S., 1960, Catalogue of the type 

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Clark, W. B., and Miller, B. L., 1912, The physiography and 

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Conrad, T. A., 1840, Fossils of the medial Tertiary of the 

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--- 1862, Descriptions of new, Recent, and Miocene 

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4, p. 722, 741-742. 

Darby, D. G., and Hoyt, J. H., 1964, An Upper Miocene 

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Mioc·ene and lower Pliocene of Virginia and North Gar~ 

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Grabau, A. W., and Shimer, H. W., 1909, North American 

index fossils-Invertebrates: v. 1, 853 p. 

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704, 13 p. 

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Hertlein, L. G., and Grant, U. S. IV, 1972, The geology and 

paleontology of the marine Pliocene of San Diego, California (Paleontology-Pelecypoda): San Diego Soc. Nat. 

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Huddesford, Gulielmus, 1770, Historiae sive synopsis methodicae Conchyliorum, et tabularum anatomicarum edit. 

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Lamarck, C. de, 1818, Histoire naturelle des animaux sans 

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Lister, Martin, 1685, Historiae sive Synopsis Methodicae 

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--- 1687, Historiae Conchyliorum Liber III, copperplate 

167. 

--- 1688, Historiae Conchyliorum Appendix ad Librum III 

copperplate 499. 

--- 1692, Appendix ad Historiae sive Synopsis Methodicae 

Conchyliorum Liber IV, copperplate 1058. 

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--- 1932, Miocene pelecypods of the Choctawhatchee formation of Florida: Florida Geol. Survey Bull. 8, 240 p. 

--- 19·36, Stratigraphic significance of Miocene, Pliocene, 

and Pleistocene Pectinidae in the southeastern United 

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--- 1943, Stratigraphy of the Miocene of Virginia and 

the Miocene and Pliocene of North Carolina, in Gardner, 

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24, 195 p. 

Page 

A 

Abbreviations used for morphological 

features ----------------- 9 

Age range ---------------------------- 1 

B 

bassleri, Chla1nys ( Lyropecten) 

•madisonius --------------- 4, 8, 9 

c 

Calvert Formation --------------- 3, 6, 8, 9,10 

Cardinal crura, obsolete ----------------- 7 

Chesapeake Group --------------------- 3 

Chesapecten, type species --------------

Chesapecten coccymelus ---------- 4, 6, 8; pl. 3, 

fig. 2; pl. 7, fig. 14 

holotype _____ pl. 3, fig.1; pl.7, fig. 15 

edgecombensis -------------------- 4,18 

jeffersonius ------- 3, 4, 6, 7, 8, 13, 18, pl. 1; 

pl. 2, figs. 1-3~ pl. 5, figs. 

3, 4; pl. 7, figs. 3-10 

lectotype --------- 15; pl. 5, figs. 5-7 

edgecombensis ----------------- 10 

madisonius _____ 4, 6, 7, 8, 10, 11, 16; pl. 6, 

figs. 1-4; pl. 7, figs. 1, 7, 8 

lectotype ------------------- 17 

middlesexensia ----------------- 6, 7, 8, 12 

lectotype ----------- 12; pl. 5, fig. 1; 

pl. 7, figs. 4, 11 

paratype ------------ 13; pl. 5, fig.2 

nefrens ____________ 4, 6, 7, 9,12, 16, 17, 18; 

pl. 2, figs. 4-6 

holotype ------------ pl. 3, figs. 4-7 

paratype ------ pl. 4, figs. 1,2; pl. 7, 

figs. 6, 13 

santamaria _____ 6, 7, 10,11,; pl. 4, figs. 5, 6 

cotypes --------------------- 12 

lectotype --------------- pl. 4, fig. 3 

paratype --------------- pl. 4, fig. 4 

sayanus --------------------------- 4 

septenarius ---------- 4, 6, 15; pl. 6, fig. 7; 

pl. 7, figs. 2, 9 

monotype ----------- pl. 6, figs. 5, 6 

Chlamys ------------------------------ 7 

jefjersonia ------------------------ 14 

madisonia ------------------------ 9, 17 

santamaria ----------------------- 11, 13 

(Chlamys) coccymelus ------------- 8 

(Lyropecten) jeffersonius --------- 13,14 

jeffersonius edgecombensis _ _ _ _ _ 17 

palmyrensis ----------------- 15 

septenarius ----------------- 15 

madisonia -------------------- 9 

madisonius -------------------- 9 

bassleri ------------------- 4, 8, 9 

richardsi ------------------- 12,13 

santamaria ------------------- 4, 11 

INDEX 

[Italic page numbers indicate descriptions] 

Page 

(Chlamys) coccymelus, Chlamys ------- 8 

coccymelus, Pecten ---------------- 8 

jeffersonius, Pecten ------- 11, 13, 14; pl. 1 

pahnyrensis, Pecten ----------- 4,15 

septenarius, Pecten ----------- 15 

madisonius, Pecten ---------------- 9,16 

santamaria m.iddlesexensis, Pecten__ 4, 12 

Choptank Formation ---------------- 3, 6,10 

clintonius, Pecten --------------------- 10,18 

coccymelus, Chesapecten ___ 4, 6, 8; pl. 3, fig. 2; 

pl. 7, fig.14 

holotype ____ pl. 3, fig.1; pl. 7, fig. 15 

Chlamys (Chlamys) --------------- 8 

Pecten (Chlamys) ----------------- 8 

Craasatella undulata ------------------- 10 

E 

Ecphora, gastropod ------------------- 3 

edgecombensis, Chesapecten ----------- 4, 18 

Chesapecten jeffersonius ----------- 10 

Chlamys ( Lyropecten) jeff ersonius _ 17 

Pecten ------------------------ 10, 16, 18 

junior synonym of P. 

m.adisonius ----------------

Pecten jeffersonius ---------------- 16 

F 

Formational names in Atlantic Coastal 

Plain -----------------------

fraternus, Pecten ---------------------

Free swimming 

G 

Geographic extent 

J 

1, 7 

13 

8 

7 

jeffersonia, Chlamys ------------------- 14 

Chlamys (Lyropecten) ------------ 13, 14 

jeffersonius, ChesaJJccten ----- 3, 4, 6,. 7, 8, 19, 

18; pl. 1; pl. 2, figs. 1-3; pl. 5, 

figs. 3, 4; pl. 7, figs. 3-10 

lectotype -------- 15.; pl. 5, figs. 5-7 

Pecten ----------------------- 3, 4, 10, 13 

Pecten (Chlamy.~) -------- 11, 13, 14; pl. 1 

cdgecombensis, Chesapecten -------- 10 

Chlamys (l.yropecten) -------- 17 Pecten _ _ __ _ __ _ __ _ __ ___ ___ _ _ ___ 16 

palmyrensis, Chlamys ( Lyropecten) _ 15 

Pecten (Chlamys) ------------ 4, 15 

septenarius, Chlamys (Lyropecten)_ 15 

Pecten------------------------ 15 

Pecetn (Chlamys) ------------- 15 

L 

Lyropecten --------------------------- 3, 7 

(Lyropecten) jefjersonia, Chlamys ----- 13,14 

jeffersonius edgecombensis, Chlamys_ 17 

Page 

(Lyropecten) jeffersonius -Continued 

palmyrensis, Chlamys ---------- 15 

septenarius, Chlamys ---------- 15 

madisonia, Chlam.ys --------------- 9 

madisonius, Chlamys -------------- 9 

bassleri, Chlamys ------------- 4, 8, 9 

richardsi, Chlan~ys ------------ 12, 13 

santamaria, Chlamys -------------- 4, 11 

M 

Macrochlamys ------------------------- 7 

madisonia, Chlam.ys ------------------- 9,17 

Chlamys (Lyropecten) ------------

madisonius, Chesapecten ----- 4, 6, 7, 8, 10, 11, 

16; pl. 6, figs. 1-4; pl. 7, 

figs. 1, 7, 8 

lectotype ------------------- 17 

Chlamys (Lyropecten) ------------ 9 

Pecten ------------- 4, 9, 10, 12, 13, 16, 18 

Pecten (Chlamys) ---------------- 9,16 

bassleri, Chlamys (Lyropecten) ---- 4, 8, 9 

richardsi, Chlamys (Lyropecten) -- 12, 13 

Mercenaria tridacnoides --------------- 3 

middlesexensis, ChesaJJecten -------- 6, 7, 8, 12 

lectotype ------ 12; pl. 5, fig. 1; pl. 7, 

figs. 4,11 

paratype ----------- 13; pl. 5, fig. 2 

Pecten (Chlamys) santamaria ----- 4, 12 

MoJl'phological discontinuities at stratigraphic breaks ----------- 6, 7 

N 

nefrens, Chesapecten ------4, 6, 7, 9, 12, 16, 17, 

18; pl. 2, figs. 4-6 

holotype ------------ pl. 3, figs. 4-7 

paratype ----------- pl. 4, figs. 1, 2; 

pl. 7, figs. 6, 13 

0 

Ontogenetic variation in Chesapecten __ 7 

Orionina vaughani 6 

p 

palmyrensis, Chlamys ( I,yropecten) 

jeffersonius --------------- 15 

Pecten (Chlamys) jeffersonius ----- 4, 15 

Pecten clintonius --------------------- 10, 18 

edgecombensis ----------------- 10, 16, 18 

fraternus ------------------------- 13 

jeffersonius ------------------ 3, 4, 10, 13 

edgecombensis ----------------- 16 

septenarius ------------------- 15 

Madisonius --------- 4, 9, 10, 12, 13, 16, 18 

senior synonym of P. 

edgecombensis -------------

septenarius ----------------------- 4, 15 

tricarinatus ----------------------- 13 

(Chlamys) coccymelus ------------- 8 

jeffersonius ____ ------ 11, 13, 14; pl. l 

palmyrensis ----------------- 4, 15 

septenarius ----------------- 15 

23 
24 

Page 

Pecten ( Chlamys) -Continued 

madisonius ----r-------------- 9, 16 

santamaria middlesexensis 4, 12 

R 

Red Bay Formation (Florida) 

richardsi, Chlamys ( Lyropecten) 

madisonius --------------- 12, 13 

s 

St. Marys Formation ------------- 3, 6, 10, 11 

Maryland and Virginia correlation _ 1 

santamaria, Chesapecten --------- 6, 7, 10, 11; 

pl. 4, figs. 5, 6 

INDEX 

Page 

santa maria, Chesapecten -Continued 

cotypes --------------------- 12 

lectotype --------------- pl. 4, fig. 3 

paratype --------------- pl. 4, fig.4 

Chlamys -------------------------- 11,13 

Chlamys (Lyropecten) ------------ 4,11 

1niddlesexe1tsis, Pecten ( Chlamys) _ 4, 12 

sayanus, Chesapecten ------------------ 4 

Sedentary life habit ------------------- 1, 8 

septenarius, Chesapecten ____ _4, 6, 15, 18; pl. 6, 

fig. 7; pl. 7,figs. 2, 9 

monotype ----------- pl. 6, figs. 5, 6 

Chlamys ( Lyropecten) jefferson ius _ 15 

Pecten --------------------------- 4,15 

Pecten Jeffersonius ---------------- 15 

Pecten (Chlamys) jeffe·rsonius ----- 15 

Stratigraphic succession --------------- 4 

Page 

T 

Tamiami Formation (Florida) 6 

Transitional forms -------------------- 6, 7 

tricarinatus, Pecten ------------------- 13 

v 

Valve convexity, change with time ----- 7 

y 

Yorktown Formation ---------------- 3, 4, 10 
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 

R.eproduction of the illustration of the earliest figured and described fossil from America. Reproduction 

is original size and from Lister's (1687) "Historiae Gonchyliorum Liher III." Lister's entire description is with each figure and not in a separate text. Say (1824) mentions this figure as being his species 

Pecten jefferson·ius. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 861 PLATE I 

4 pec-ten onuuun1. lo1zs~ ntaxun.UJ, 111uuLS cnAvu.t. Dc.J.o ucl dcr.C-~11.- ad Suntnu..~.~ . S't.r....i.~s don.alu..s, 

rictn 11lu~. t'a~-z-n/, c.u.lt . fr~_./Lu"Ld,: ~ 
PLATE 2 

FIGURES 1-3. Chesapecten jeffersonius (Say) ( p. 13). 

Ventral, posterior, and anterior views of articulated specimen USNM 193445, from 

Cobham Wharf, Va. (USGS loc. 25331), length 135.0 mm, height 144.1 mm. 

4-6. Chesapec.ten nefrens n. sp. (p. 9) . 

Ventral, posterior, and anterior views of articulated specimen USNM 193443, from 

Camp Conoy, Md. (USGS loc. 25299), length 134.3 mm, height 113.9 mm. 
GEOLOGICAL SU RVEY l'!WFESSIONAL PAPER R6 1 PLATE 2 

6 

CHESAPECTEN JEFFERSONTUS (SAY) AND C. NEFRENS N. SP. 
PLATE 3 

FIGURES 1, 2. Chesapecten coccymelus (Dall) (p. 8). 

1. Left valve, USNM 87754, holotype from Plum Point, Md. (USGS loc. 25294), length 

24.3 mm. 

2. Left valve, USNM 193440, from Camp Roosevelt, Md. (USGS loc. 25295), length 

38.0 mm. 

3. Chesapecten sp. 

Right valve, USNM 193441, from Camp Roosevelt, Md. (USGS Joe. 25295), length 35.2 

mm. 

4-7. Chesapecten nej?·ens n. sp. (p. 9). 

Right anrd left valves of articulated specimen, holotype, USNM 193442, from Camp 

Conoy, Md. (USGS Joe. 25299), length 177.4 mm. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 861 PLATE 3 

1 

2 

4 5 

6 7 

CHESAPECTEN COCCYMELUS (DALL), C. SP., AND C. NEFRENS N. SP. 
PLATE 4 

FIGURES 1, 2 . Chesapecten nefrens n. sp. (p. 9). 

Right and left valves of articulated specimen ( USNM 193443), para type, from Camp 

Canoy, Md. (USGS Joe. 25299), length 134.3 mm. 

3-6. Chesapecten santamaria (Tucker) ( p. 11). 

3. Right valve, lectotype, USNM 193448, from St. Marys River, Md. (USGS Joe. 

2342), height 124.7 mm. 

4. Left valve, paratype, USNM 2498, from St. Marys River, Md., length 123.6 mm. 

5, 6. Right valve, USNM 193444, from Essex Mill, Va., (USGS Joe. 25058), length 

129.0 mm. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 861 PLATE 4 

1 2 

3 4 

5 

6 

CHESAPECTEN NEFRENS N. SP. AND C. SANTAMARIA (TUCKER) 
PLATE 5 

FIGURES 1, 2. Chesapecten middlesexensis (Mansfield) (p. 12). 

1. Right valve, lectotype, USNM 373074, from Urbanna, Va. (USGS loc. 3915), 

length 140.7 mm. 

2. Left valve, paratype, USNM 207693, from Urbanna, Va. (USGS loc. 3915), 

length 154.1 mm. 

3-7. Chesapecten jefferson ius (Say) (p. 13). 

3, 4. Right and left valves, articulated specimen, USNM 193445, from Cobham Wharf, 

Va. (USGS loc. 25331), length 135.0 mm. 

5-7. Anterior, left, and right views of articulated specimen, lectotype, British Museum No. L13212, from Virginia, length 99.2 mm, height 93.1 mm. 
GEOLOGICAL SURVEY PIWFESSIONAL PAPEK 861 PLATE 5 

6 

7 

CHESAPECTEN MJDDLESEXENSIS (MAN8FIELD) AND C. JEFFERSON/US (SAY) 
PLATE 6 

FIGURES 1-4. Chesapecten madisonius (Say) (p. 16). 

1, 2. Right valve, USNM 193447, from Cobham Wharf, Va. (USGS loc. 25336), 

length 112.8 mm. 

3, 4. Left valve, lectotype I ANSP 31787), from Virginia, length 123.9 mm. 

5- 7. Chesapectenseptenarius (Say) (p. 15). 

5, 6. Left valve, holotype, British Mus,eum No. L13197, from Virginia, length 64.9 mm. 

7. Right valve, USNM 193446, from Lee Creek mine, North Carolina (USGS loc. 

25334), length 116.5 mm. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 861 PLATE 6 

4 

5 7 

CHESAPECTEN MADISONIUS (SAY) AND C. SEPTENARIUS (SAY) 
PLATE 7 

FIGURES 1, 7, 8. Chesapecten madisonius (Say) (p. 16). 

1, 8. Right valve, USNM 193447, from Cobham Wharf, Va. (USGS loc. 25336). 

View of auricles (length of auricles along line of ligament 59.7 mm) and enlargement of 20 mm-long section of exterior sculpture near anterior margin. 

7. Right valve, juvenile, British Museum No. LL27417, from the collection of John 

Finch, length of auricles along line of ligament 28 mm. 

2, 9. Chesapecten septenarius (Say) ( p. 15). 

Right valve, USNM 193446, from Lee Creek mine, North Carolina (USGS Joe. 

25334). View of auricles (length of auricles along line of ligament 60.6 mm) and 

enlargement of 20 mm-long section of exterior sculpture near anterior margin. 

3, 10. Chesapecten jefjersonius (Say) (p. 13). 

Right valve, USNM 193445, from Cobham Wharf, Va. (USGS Joe. 25331). View of 

auricles (length of auricles along line of ligament 80.6 mm) and enlargement of 

20 mm-long section of exterior sculpture near anterior margin. 

4, 11. Chesapecten middlesexensis (Mansfield) (p. 12). 

Right valve, lectotype, USNM 373074, from Urbanna, Va. (USGS Joe. 3915). View 

of auricles (length of auricles along line of ligament 68.3 mm) and enlargement 

of 20 mm-long section of exterior sculpture near anterior margin. 

5, 12. C hesapecten santamaria (Tucker) ( p. 11) . 

Right valve, USNM 193444, from Ess,ex Mill, Va. (USGS Joe. 25058). View of auricles (length of auricles along line of ligament 54.5 mm) and enlargement of 20 

mm-long section of exterior sculpture near anterior margin. 

6, 13. Chesapecten nefrens n. sp. (p. 9). 

Right valve, paratype, USNM 193443, from Camp Conoy, Md. (USGS Joe. 25299). 

View of auricles (length of auricles along line of ligament 73.2 mm) and enlargement of 20 mm-long section of exterior sculpture near anterior margin. 

14, 15. Chesapecten coccymelus (Dall) (p. 8). 

14. Right valve, USNM 207692 from Camp Roosevelt, Md. (USGS loc. 25295). View 

of auricles (length of auricles along line o,f ligament 32.8 mm). 

15. Left valve, holotype, USNM 87754, from Plum Point, Md. View of auricles 

(length of auricles along line of ligament 14.2 mm). 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 861 PLATE 7 

2 

3 

6 

8 

11 

12 

13 

15 

CHESAPECTEN MADTSONIUS, C. SEPTENARIUS, C. JEFFERSON I US, C. MIDDLESEXENSIS. 

C. SANTAMARIA, C. NEFRENS, AND C. COCCYMEL US 

"If U.S. GOVERN MENT PRINTING O F FICE: 1975 0 - 585-47 1 /33