PALAEONTOLOGY OF NEW-YORK.' 
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Discina seneca (n. s.). 
PLATE II. 
Dorsal valve broadly oval; apex about one-third from the posterior 
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. 
Surface marked by concentric, somewhat regular folds or wrinkles 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 striae of Discina. 
Two specimens of dorsal valves only have been seen : they have the same form 
as D. media and D. lodensis; bnt the apex is much more elevated, and the surface 
striae are not of the same fine and regular character, though, the specimens 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 al¬ 
ways flattened, while both the old and young of this maintain their proportions 
as described. With the single larger specimen, I hesitated to characterize 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 formation and locality. In the upper part of the Hamilton group : 
on the east shore of Seneca lake. 
Discina media (n. s.). 
PLATE II. 
Compare Discina lodensis ) pa. 22, pi. 2. 
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, directly in the longitudinal axis of 
the shell, or often a little oblique. 
