Chesapecten skiptonensis (Mansfield, 1936)
MANSFIELD, W. C. 1936. Stratigraphic significance of Miocene, Pliocene and Pleistocene Pectinidae in the Southeastern United States. Journal of Paleontology, 10 (3): 168-192. [p. 186, pl. 23, fig. 12]
1936 Pecten (Chlamys) skiptonensis Mansfield, 1936
W. C. Mansfield, 1936, plate 23.
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«Shell small, nearly equilateral, compressed but not flat. Right anterior auricle with a deep byssal sinus and sculptured with four fine radial threads over the surface between the dorsal margin and the byssal area. Byssal area crossed with fine threads which surmount a rather strong thread parallel with and adjacent to the submargin. Posterior ear marked with four weak radials. Disk sculptured with six ribs, the anterior and posterior weak, others much wider, equally and widely spaced. Faint growth lamellae ornament the ribs and interspaces. Obscure subradials visible only at the distal ends of the ribs, none observed between them. The ctenolium provided with fine short spines.
Dimensions of holotype: Right valve (U. S. Nat. Mus. 143658), length, 8.5 mm.; height, 10 mm.; convexity 1.6 mm. Pecten (Chlamys) skiptonensis differs from P. (C.) rogersi Conrad in lacking strong radial sculpture over the whole shell. Type locality: Station 2366, near Skipton, Talbot County, Maryland. Horizon: Choptank formation (?)» WENDELL CLAY MANSFIELD, 1936
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«Discussion.— The immature specimen that is the holotype of Chesapecten (?) skiptonensis Mansfield was found in Talbot County on the Eastem Shore of Maryland and was assigned to Pecten rogersi Conrad by Dall (1898b), Glenn (1904), and Rowland (1936). It superficially resembles "P." rogersi in that they both have a few wide ribs, but differs from it in its radial sculpture, shell stn¡cture, rib shape, and disk margins. "P." rogersi is known to occur only in the Rushmere Member of the Yorktown Formation (upper Pliocene) in Virginia. The specimen that Mansfteld used for a holotype (USNM 143658) was an immature individual, only 10 mm in height, but he considered it distinct enough, even at that stage, to be separable from "P." rogersi Conrad. The specimen ftgured here is a somewhat more mature individual collected in 1919 by William Palmer from Bed 14 ("Zone" 14) in the Calvert Formation south of Chesapeake Beach, Calvert County, Md. The figured specimen is a right valve, small (height 39 mm, length 36.4 mm, width 5.3 mm), slightly higher than long, with 6 pronounced ribs with wider interspaces. Anterior and posterior ribs small, the other four large and flat. Very feeble development of 3 radial lirae on the ribs up to a height of 10 mm. After that stage, 7-9 slightly more prominent, but still subdued, radial lirae develop on the ribs and the interareas. Fine, platelike incrementals may be seen in concentric rows between the lirae. The ribs show a somewhat interrupted pattem in that they change direction to form a very slight zigzag pattern. This feature is reminiscent of Nodipecten, but only low undulations can be seen, not distinct nodes. Auricles large with deep byssal notch. Fine scabrous lirae radiating from the beak. Byssal fasciole narrow and deep. Four very small teeth on the active ctenolium. Interior of valve deeply ribbed to notch exterior sculpture. Valve relatively shallow. Muscle attachment area partly broken and not distinct.
In general outline and rib numbers, C. (?) skiptonensis resembles Anatipopecten, but the auricles are distinctly sharper in that taxon. Until sufficient numbers of C. (?) skiptonensis are found, its true taxonomic placement will be in question. Type information.— Holotype: USNM 143658. Type locality: Near Skipton, Talbot Counyy, Md. USGS locality 2366). Figured specimen.— Right valve (USNM 405210), from Bed 14 of the Calvert Beach Member of the Calvert Formation, just downbay of Chesapeake Beach, westem shore of the Chesapeake Bay, Calvert County, Md. Collected by William Palmer, July 1919. Stratigraphic and geographic range.— Exact stratigraphic position for the holotype is unknown but is probably the same as the figured specimen: Calvert Formation, Calvert Beach Member, Bed 14 (lower middle Miocene) in Maryland. It is known only from Calvert and Talbot Counties, Md.» 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. 73]
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Chesapecten (?) skiptonensis (Mansfield, 1936); L. W. Ward, 1992, Molluscan bioestratigraphy of the Miocene, middle Atlantic Coastal Plain of North America, plate 18, figures 5, 8.
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