DISCING OF THE HAMILTON GROUP. 
21 
Surface finely and evenly striated by fine 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 margin, 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 cha¬ 
racters, and that we have, only a well-marked variety of that species. The species 
is not uncommon in the Hamilton group, both in the bluish shales and in the 
dark slaty beds, which are of the same character as the Genesee slate. 
A single specimen of a ventral valve, found in the upper part of the Chemung 
group in Steuben county, possesses characters so entirely similar to those of the 
Hamilton group, that I can find no means of separating it. 
.The oval form and excentric position of the apex are sufficient to distinguish 
the species from others of these groups. The elliptical specimens measure half 
an inch, or more, in length, with a width of nine-twentieths of an inch. 
Geological formation and locality. In the Hamilton group, on the shores of 
Seneca lake near Ovid, and in the upper part of the group on the shores of the 
Canandaigua lake; and in the green shale of the Chemung group in the south 
part of Steuben county, at Troupsburgh. 
Discina (sp.?). 
PLATE II. 
A small nearly circular specimen of a ventral valve, occurring in the 
Marcellus slate, is marked by fine crowded and scarcely distinct striae : 
the apex is excentric, being scarcely more than one-third the length of 
the shell from the posterior margin; with a short, narrowly oval 
foramen. 
This specimen is unlike any other one before me, and maybe a distinct species; 
but I hesitate to designate it without more material. 
Geological formation and locality. In the Marcellus slate : near Bridgewater, 
N.York. 
