Christinapecten Ward, 1992
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, pls. 1-26. [p. 73]
«Subgenus Christinapecten, new subgenus
Diagnosis.— Subequivalved, both valves convex, the left deeper than the right. Sculpture consists of 20 small ribs which bifurcate ar an early stage. A single riblet appears in the interstices and becomes nearly as prominent as the primary ribs. Auricles large, scaly, with a deep byssal notch.
Description.— Subequivalved, both valves convex, with the right valve rltghdv convex, and the left moderately so, especially in the umbonal area. Disk oudine nearly ovate, with large auricles. Right valve with small posterior auricle and large anterior one with deep byssal notch. Byssal fasciole sharply deftned with ffve (in the type species) denticles prominent in the active ctenolium. Left valve with small, sloped posterior auricle and a larger anterior one with a small byssal sinus. Exterior sculpture consists of twenty small ribs radiating from a sharp beak. Ribs bifurcate when disk approximarely 10 mm in height to form two small, smooth riblets. A single riblet appears in the interstices at the same stage and becomes nearly as prominent as the primary ribs. Other very small subordinate riblets appear in interstices at late growth stage. Radial sculpture consists of very small, plate-like incrementals between the riblets. Interiorly, the valves marked by very shallow ribs, which reflect the exterior inter-rib areas, and which are obscured in the upper one-half of the valves. Ligamental pit small and triangular. Muscle scars weakly impressed and indistinct.
Discussion.-- This taxon is based on Chesapecten (Christinapecten) marilandica, which is common in rhe
Drumcliff Member of the Choptank Formation. It has been treated as a species of Placopecten but is distinctly different from that genus in its ribbing, shell outline, auricles, and shell structure. More recently Glibert and van de Poel (1965) placed it in the genus Mimachlamys, a Tasmanian form, but that taxon is strongly ribbed and covered with prominent scales. Christinapecten has the distinct outline and prominent auricles of Chesapecten, but it is distinguished by fine bifurcating ribs and interradials that are nearly devoid of scaling. Christinapecten consistently has more ribs than Chesapecten sensu stricto, having 20 primary ribs, whereas Chesapecten has between 13 and 17. The first forms of this subgenus are known from Bed 10 ("Zone" 10 of Shattuck, 1904) of the Plum Point Marl Member of the Calvert Formation and are unnamed. They differ from C. (C.) marylandica in having three small riblets on the primary ribs instead of two but in other respects are very similar. Type information.— Type species: Based on Pecten marylandicus Wagner, 1839, the neotype of which is USNM 405211.
Stratigraphic and geographic range.— Calvert Formation, Plum Point Marl Member, Bed 10 (lower middle Miocene) in Maryland; Calvert Beach Member (lower middle Miocene) in Maryland and Virginia. Choptank Formation, Drumcliff Member, Bed 17 (middle middle Miocene) in Maryland.»
LAUCK W. WARD, 1992
|
Chesapecten (Christinapecten) marylandica (Wagner), neotype, right valve; L. W. Ward, 1992, Molluscan bioestratigraphy of the Miocene, middle Atlantic Coastal Plain of North America, plate 16, figure 1.
|