Family PECTINIDAEA number of Maryland Miocene species have in the past been referred to the genus Pecten, but only Pecten humphreysii Conrad (Pl. 7, fig. 3, 4) is accurately so identified. It occurs in the lower part of the Plum Point Marl (zones 5 through 10 of Shattuck). It is characterized by having one flat and one convex valve. The convex valve has six to eight broad flat radiating ribs with the inter-rib areas about one-half the width of the ribs. The flat valve has ribbing that occupies the inter-rib interval of the other and wide interspaces equivalent to the ribs of the convex valve. This species is usually not common, but the writer once found a large number in a bed a few feet above the Pycnodonte percrassa zone a short distance west of Davidsonville in Anne Arundel County.
The common pectens in the Maryland Miocene belong to the genera Placopecten and Chesapecten. One of the most abundantly represented and characteristic species of the Calvert and Choptank Formations is Chesapecten nefrens Ward and Blackwelder (Pl. 7, fig. 6). It is characterized by 13 to 18, usually about 16, radial ribs, with the entire surface of the valve overlain by finer scaly riblets. Usually there are about three riblets on the tops of the major radial ribs, and they tend to be more strongly developed than are the riblets in the spaces between the major ribs. The species is very variable, however, particularly in the strength of the development of the riblets. One form, in which the middle of the three riblets on each rib is much more strongly developed than the other two is the variety bassleri Tucker-Rowland, which occurs only in the Calvert Formation. In a more extreme development, in Chesapecten coccymelus (Dall), a rather rare species of the Calvert fauna, the finer riblets are all lost except for the strong central one on each major rib.
Typical specimens of Chesapecten nefrens attain diameters up to six inches in the Choptank Formation, especially near the top of the Drumcliff Member (zone 17) where they occur in vast numbers, often with large specimens of the barnacle Balanus concavus Bronn (Pl. 29, fig. 16-19) attached to them. The adult specimens in the Calvert Formation are almost always of smaller size than those of the Choptank. The middle of the three strong riblets on the ribs appears first in the development of the individual. Small specimens, an inch or less in diameter, have a superficially very different appearance from the adults. The relationship is apparent on comparing them with the umbonal region of a well preserved adult specimen.
In the St. Marys Formation Chesapecten santamaria (Tucker) (Pl. 7, fig. 7) is the abundant pectinid species. It is characterized by 12 to 14 rather broad radial ribs that are usually slightly wider than the interspaces between them. The surface of the ribs and the interspaces are marked by many relatively fine scaly riblets. These are not as strong as the riblets in C. nefrens and are not more strongly developed on the surfaces of the ribs than they are in the interspaces, rather there is a tendency for the reverse to be true with a more prominent riblet often in the middle of an interspace. In the Maryland Miocene Volume this species was identified and illustrated as "Pecten" jeffersonius Say, a species of the Yorktown Formation that is characterized by the presence of about nine strong flattened radial ribs. I have seen no undoubted specimens of jeffersonius from the St. Marys Formation.
Another species, Placopecten marylandicus (Wagner) (Pl. 7, fig. 5), occurs in the Choptank Formation and appears to be confined to the Drumcliff Member (zone 17). This species, having a diameter of about three inches, is smaller than the Choptank specimens of C. nefrens. The valves are ornamented by radial lines that tend to group themselves into major ribs of varying degrees of prominence. There is a fine concentric ornamentation in the spaces between the radial lines, but it does not cross them.
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