CONTRIBUTIONS TO PALEONTOLOGY. 27 



Surface marked by concentric, somewhat regular folds or wrin- 

 kles in the larger specimens, and, in a smaller one, the surface 

 is similar, but less uneven ; the concentric markings being more 

 like undulations of the surface, than the ordinary stride of Di- 



SCINA, 



Two sp<3cimens of dorsal valves only have been seen : tliey have the same 

 form as D. media and D. lodeiisis ; but the apex is much more elevated, 

 and the surface striae are not of the same fine and regular character, though, 

 the s,pecimens being partial casts, this feature may be somewhat obliterated. 

 One of the specimens has a length of six-tenths of an inch, with a width 

 of five-tenths ; while the smaller one has a length of one-fifth, and a width 

 of one-sixth of an inch. 



These specimens occur in the same shale with the other species, which 

 are always flattened, while both the old and young of this maintain their 

 proportions as described. With the single larger specimen, I hesitated to 

 characteri-ze the species ; but finding a small one of the same form and 

 character, I can scarcely indicate it as a variety of either of the other 



species which it resembles in general form, • 



Geological formatio7i and locality. In the upper part of the Hamilton 

 group : on the east shore of Seneca lake. 



'* DISCINA MEDIA ( n. s,)^" 

 Compare Discina lodensis. 



Shell broadly elliptical or subcircular, variable in form. Dorsal 

 valve very depressed-convex* apex excentric, pointed, and 

 inclined towards the posterior border. Ventral valve flat, or a 

 little convex just anterior to the foramen ; foramen narrow, 

 diretly in the longitudinal axis of the shell, or often a little 

 oblique. 



Surface finely and evenly striated by the regular elevated striae, 

 distant from each other more than twice their width. The apex 

 of the dorsal valve is about one-third, and sometimes less than 

 one-third the length of the shell from the posterior margin. 

 Perforation of the ventral valve narrowly oval or sublinear, 

 about one-third the length of the shell from the posterior mar- 

 gin, and extending towards the edge of the shell. 



This species resembles in form the D. lodensis of the Genesee slate ; but 

 it is more coarsely and distantly striated, and does not present the radiating 

 folds or undulations observed in that species. The position and relations of 

 the apex and foramen are similar to those of the D. lodensis; and it is 

 possible that conditions of the sediment, and other physical causes, may 

 have affected the external characters, and that we have only a well-marked 

 variety of that species. The species is not uncommon in the Hamilton group, 



