LAMELLIBRANCHIATA FROM BASE OF UPPER MARLS. 211 



posterior extremity, whicli is narrow and obliquely truncate. Basal margin 

 rather long-, straight, or perceptibly hollowed; anterior end short, rapidly 

 declining from the beaks to below the middle of the height, where it has a 

 somewhat tumid appearance. Valves somewhat convex, most elevated on 

 the umbones and slightly sinuous just anterior to the umbonal ridge. This 

 latter feature is somewhat subdued and rounded, and the cardinal slope 

 narrow. On the internal casts the muscular scars are rather small and not 

 so distinctly marked as on most species, and on the cardinal slope there is 

 present a faint ridge between the umbonal angle and the cardinal margin. 



The above features are taken from internal casts. I have not seen the 

 shell, but Mr. Gabb fig'ures it in the Journal of the Academy of Natural 

 Sciences above cited, and gives it as strongly marked by concentric undu- 

 lations, while in general form it accords perfectly with the form of the casts 

 which I have seen. Mr. Gabb says it differs from C. Monmouthensis in its 

 greater proportional length, smaller umbones, less distinctly marked umbo- 

 nal ridge, which is lost in the general curvature of the shell before reaching 

 the posterior extremity, and in the secondary I'idge, which is seen on the 

 cardinal slope in the internal cast. There is a very close general resem- 

 blance in form, however, between the two species, but still they are suffi- 

 ciently distinct to be readily recognized, even in the condition of internal 

 casts. 



Formation and locality. — The internal casts, which are quite numerous in 

 the State collection, are mostly from the pits of John Irons, New Egypt, and 

 other places near by, and also from Poke Hill, and are marked as coming 

 from the lower layer of the " third bed of marl," while Mr. Gabb's specimens 

 were irom the deep cut on the Delaware and Chesapeake Canal, in the State 

 of Delaware, which would be from the Lower Marl Bed. It is possible the 

 species may have this great vertical range, but if so it is very different in 

 this respect from most of the associated species, and especially those of this 

 genus. All specimens, however, which I have seen with authentic localities 

 have been from the base of the Upper Marls. 



