LAMELLIBEANCHIATA OF THE LOWER MARLS. 31 



Ostrea plumosa. 



Plate III, Figs. 12 and 13. 



Ostrea plumosa Morton. Sj'nopsis, p. 51, PI. Ill, Fig. 9. Gabb, Synop., p. 153. P. A. 

 N. Sci., 1876, p. 320; Meek, Check-list, p. 6. Geol. N. Jersey, 1868, p. 724, 

 and others. 



Shell small, ovate, ovate-triangular or elongete-spatulate, thin and some- 

 what fragile irregularly convex on the upper valve, often subangulated 

 longitudinally, either along one side or the other, beak of the upper valve 

 thin, sharp, and pointed; the ligamental area small and inconspicuous in 

 most cases, though sometimes of moderate size. Exterior of the upper 

 valve marked by obscure plications in all the type specimens, which cross 

 the valve obliquel}^ in either direction from right to left or oppositely; also 

 by fine radiating striae which obscurely diverge from a more or less median 

 line and pass toward the margin on either side. On the interior tlie margin 

 of the valve near the apex is more or less crenulate. The muscular scar is 

 small and lateral. Lower valve not yet observed. 



The type specimens used by Dr. Morton furnish the above characters, 

 his figured specimen being one of them, but he appears to have considered 

 them as lower valves, while they are all upper valves, as the position of the 

 muscular scar and the ridge on the ligamental area would indicate, as well 

 as the obliquity of the valve. Among the collections which I have examined 

 I have observed no lower valves which I can identify with them. Mr. Meek 

 mentions in his Invert. Pal. Geol. Surv. Territ., however, that the lower valve 

 is usually attached by nearly its entire surface. The plications which cross 

 the upper valves obliquely are probably the effects of marking on some other 

 shell, possibly Exogyra costata, on which the oyster has grown, and not 

 an organic feature of the species. This I think is plain from the fact that 

 they cross the shell in opposite directions in diff"erent specimens. Ostrea 

 cretacea Morton, Synop. p. 52, PI. XIX, Fig. 3 is a very closely allied, if 

 not identical species. It is also often obscurely jjlicated and striate, the size 

 and position of the muscular scar are tlie same, but the apex of the valve 

 is proportionally a little wider in the specimens which I have examined. 

 This, however, can scarcely be considered as a specific feature in shells of 

 •such variable characters as oysters. A single individual in the same collec- 



