DISCING OF THE HAMILTON GROUP. lA 



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 striee 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, 

 which in its greatest diameter is four inches. 



Geological formation and locality. In the arenaceous shales of the Hamilton 

 group : near Schoharie. 



Discina duria ( n. s.). 



PLATE IL 



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. 



SuKFACE marked by fine concentric striae, and the cast by folds or wrin- 

 kles in the same direction. 



This species closely resembles the D. newberryi, 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 stri^ on the cast, which are 

 probably caused by the soft parts of the animal, cannot be relied upon as cha- 

 racteristic. 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 questioned, but 

 for the preservation of four individuals (one of them a ventral valve) upon a 

 specimen of Pleurotomaria sulcomarginata. The same species occurs in Canada 

 West, attached to Spirifer. 



Geological formation and locality. In the Hamilton group : at Hamilton ; on 

 the east shore of Seneca lake, N.Y. ; and in Canada West. 



