Chesapecten nefrens Ward & Blackwelder, 1975
WARD, L. W. & B. W. BLACKWELDER. 1975. Chesapecten, a new Genus of Pectinidae (Mollusca:Bivalvia) from the Miocene and Pliocene od Eastern North America. Geological Survey Professional Paper, 861: 1-24, pls 1-7. [p. 9, pl. 2, figs. 4-6; pl. 3, figs. 4-7; pl. 4, figs. 1-2; pl. 7, figs. 6-13]
1975 Chesapecten nefrens Ward & Blackwelder, 1975
L. W. Ward & B. W. Blackwelder, 1975, plate 2.
L. W. Ward & B. W. Blackwelder, 1975, plate 3.
L. W. Ward & B. W. Blackwelder, 1975, plate 4.
L. W. Ward & B. W. Blackwelder, 1975, plate 7.
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«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. Illustrated 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, 128º. 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 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 nefrens from map locality 7 (USGS loc. 25299) (see table 3 for explanation of the abbreviations) :
TABLE 3.— Morphological feature abbreviations used in measurement of specimens
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 nefrens also occurs in the following Maryland middle Miocene zones of Shattuck:
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 nefrens 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 clintonius. 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. jeffersonius 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 Natural 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 madisonius 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.» LAUCK W. WARD & BLAKE W. BLACKWELDER, 1975
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«Discussion.— Chesapecten nefrens differs from C. covepointensis and C. santamaria, both in the St. Marys Formation, in having narrower, unflattened ribs, larger gapes between disk flanks, larger gapes along the ventral commissures, coarser scabrous lirae, a larger byssal notch, and larger auricles. Specimens of C. nefrens that occur stratigaphically from Bed 14 ("Zone" 14 of Shattuck, 1904) through Bed 17 ("Zone" 17, the Drumcliff Member of the Choptank Formation) and Bed 18 ("Zone" 18, the St. Leonard Member of the Choptank Formation) are somewhat narrower in disk oudine than C. nefrens from Bed 19 ("Zone" 19, Boston Cliff Member of the Choptank Formation). Adult specimens from the Boston Cliffs Member, from which the holotype came, are markedly elongate and have a correspondingly long hinge and wide auricles. The earlier forms of C. nefrens, in the Calvert Beach (Calvert Formation) and Drumcliff Members, are more ovate and possess proportionately smaller auricles. This difference may only be ecophenotypic but it is consistent enough to be of biostratigraphic value.
Type information. Holotype: USNM 199442. Type locality: Boston Cliffs Member (Bed 19) at Camp Conoy below the Baltimore Gas and Electric Atomic Power Plant, Calvert Counyy, Md. (USGS locality 25299). Figured specimens.— Plate 15, figure 4: Holotype (USNM 193442) from the type locality. Plate 16, figure 2: Rightvalve (USNM 405202) fromDrumcliff on the Patuxent River, St. Marys County, Md. (locality 16). Stratigraphic and geographic range.— The usual form of C. nefrens occurs stratigraphically from Beds 14 (Calvert Beach Member of the Calvert Formation) (lower middle Miocene) to 19 (Boston Cliffs Member) (upper middle Miocene) of the Choptank Formation. Several species of Chesapecten in Bed 10 of the Calvert Formation (lower middle Miocene) appear to be closely related but never attain the size of the normal adults in the upper Calvert and Choptank Formations. Chesapecten nefrens occurs in the Calvert and Choptank Formations in Maryland and Virginia. In Virginia, C. nefrens is found in the Calvert Beach Member and the Boston Cliffs Member at the Nomini Bluffs on the Potomac River, Westmoreland County and in the Calvert Beach Member at Fones Cliffs on the Rappahannock River, Richmond County.» WARD, L. W. 1992. Molluscan bioestratigraphy of the Miocene, middle Atlantic Coastal Plain of North America. Memoirs of the Virginia Museum of Natural History, 2: 1-159. [p. 71]
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Chesapecten nefrens Ward & Blackwelder, 1975; L. W. Ward, 1992, Molluscan bioestratigraphy of the Miocene, middle Atlantic Coastal Plain of North America, plate 15, figure 4 (above); plate 16, figure 2 (below).
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