26 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



The single specimen known is a partially exfoliated exterior surface of a 

 ventral valve : the margin of the anterior portion is somewhat broken and 

 worn away. Certain appearances indicate that the dorsal lies beneath this 

 ventral valve, imbedded in the matrix, and the two pressed closely together. 



This species is a large and remarkable form ; the transverse diameter is 

 nearly three inches, and from the apex to the posterior margin is one inch 

 and a half. The anterior side being broken off, we infer, from the ordinary 

 proportions of these shells, that it has been at least as long as -the posterior 

 part, which gives a longitudinal diameter of three inches. The concentric 

 striae are stronger than in any of the other species in these rocks, and the 

 strong radiating ridges are a very distinguishing feature. 



This is the largest well-authenticated Discina that I know ; though I 

 have a discoid fossil under consideration, possessing the general appearance 

 of DisciNA, and which in its greatest diameter is four inches. 



Geological formation and locality. In the arenaceous shales of the Ha- 

 milton group : near Schoharie. 



♦' DISCINA DORIA ( n. s.)." 



Shell subcircular or oblate, the transverse diameter usually the 

 greater. Dorsal valve convex ; apex elevated, subterminal. 

 Ventral valve flat or concave, the apex excentric ; foramen 

 comparatively large, oval, with margins depressed. Shell thin. 



Surface marked by fine concentric striae, and the cast by folds 

 or wrinkles in the same direction. 



This species closely resembles the D. newherryi, from Cuyahoga falls, 

 Ohio ; but that shell is somewhat thicker and stronger, with the apex of the 

 dorsal valve more elevated. The specimens of the present species observed 

 are likewise smaller. The presence or absence of the radiating striae on the 

 cast, which are probably caused by the soft parts of the animal, cannot be 

 relied upon as characteristic. The length of the largest specimen is about 

 three-tenths of an inch, with a width of seven-twentieths of an inch. 



The most characteristic specimens of this species which have been seen, 

 are adhering to other fossils ; and its identity with DisciNA might be que- 

 stioned, but for the preservation of four individuals (one of them a ventral 

 valve) upon a specimen of Plcurotomaria sulcomarginata. The same species 

 occurs in Canada West, attached to Spirifer. 



Geological formation and locality. In the Hamilton group : at Hamil- 

 ton, Madison county ; on the east shore of Seneca lake ; and in Canada 

 West. 



" DISCINA SENECA ( n. s.)." 



Dorsal valve broadly oval ; apex about one-third from.the poste- 

 rior margin, and, in a specimen six-tenths of an inch in length, 

 is elevated one-tenth of an inch above the plane of the margins. 



