Chesapecten Ward & Blackwelder, 1975
WARD, L. W. & B. W. BLACKWELDER. 1975. Chesapecten, a new genus of Pectinidae (Mollusca: Bivalvia) from the Miocene and Pliocene of eastern North America. United States Geological Survey Professional Paper, 861: 1-24 [p. 7]
«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 nefrens Ward 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 comparisons.— 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 Macrochlamys 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 jeffersonius. 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:
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.»
LAUCK W. WARD & BLAKE W. BLACKWELDER, 1975
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Chesapecten nefrens n. sp.; L. W. Ward & B. W. Blackwelder, 1975, Chesapecten, a new genus of Pectinidae (Mollusca: Bivalvia) from the Miocene and Pliocene of eastern North America, plate 2, figures 4-6; plate 3, figures 4-6.
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Chesapecten middlesexensis hunterae n. ssp.; T. R. Waller, 2018, Systematics and biostratigraphy of Chesapecten and Carolinapecten (Mollusca: Bivalvia: Pectinidae) in the upper Miocene and Pliocene "lower Tamiami Formation" of southwestern Florida, figure 3.
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«Family PECTINIDAE RAFINESQUE, 1815
Subfamily PECTININAE RAFINESQUE, 1815 Tribe AEQUIPECTININI NORDSIECK, 1969 Genus CHESAPECTEN WARD AND BLACKWELDER, 1975 Chesapecten Ward and Blackwelder, 1975:7; Gibson, 1987:69; Campbell, 1993:25.
Chesapecten (Chesapecten) Ward and Blackwelder, 1975. Ward, 1992b:68. Type species.— Chesapecten nefrens Ward and Blackwelder, 1975, by original designation; Choptank Formation, middle Miocene, zone 19 of Shattuck (1904), Chesapeake Bay cliffs, Calvert County, Maryland.
Included species.— coccymelus (Dall, 1898); covepointensis Ward, 1992b; jeffersonius (Say, 1824); madisonius (Say, 1824); middlesexensis (Mansfield, 1936); monicae Ward, 1992b; nefrens Ward and Blackwelder, 1975; nematopleura (Gardner, 1936), santamaria (Tucker, 1934); sayanus (Dall, 1898); and septenarius (Say, 1824).
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.” (Ward and Blackwelder, 1975:40).
Geographic range.— New Jersey to Florida.
Stratigraphic range.— Kirkwood Formation of New Jersey and Delaware, lower Miocene, Burdigalian (Ward, 1998:80) to upper Yorktown Formation,
Zone 2 of Mansfield (1944), upper Pliocene, Piacenzian.» WALLER, T. R. 2011. Systematics and biostratigraphy of Chesapecten and Carolinapecten (Mollusca: Bivalvia: Pectinidae) in the upper Miocene and Pliocene "lower Tamiami Formation" of southwestern Florida. Bulletin of the Florida Museum of Natural History, 56 (1): 1-47, figs. 1-11. [p. 11]
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