TREMATOSPIRA OF THE HAMILTON GROUP. 
273 
Dorsal valve gibbous, more convex than the opposite : beak abruptly 
incurved, and concealed within the cavity of the ventral valve. Mesial 
fold more or less conspicuous, sometimes scarcely raised above the 
adjacent plications. 
Surface marked by nine or ten strong angular plications : three of these, 
in the centre of the dorsal valve, are more approximate and smaller 
than the others, giving the character of a mesial fold which is some¬ 
times scarcely elevated above the general contour of the surface; 
while the two central plications on the opposite valve are much smaller 
than the adjacent ones, and abruptly depressed. The concentric 
lamellae of growth, in crossing the plications, give a series of zigzag 
lines ; and these often become crowded and very conspicuous on the 
front of the shell. Entire surface finely granulose, and the shell struc¬ 
ture punctate. 
The interior of the dorsal valve shows a bilobed cardinal process, with 
obscure indications of the crural bases. There is a distinct mesial septum 
in the .upper part of the valve. The muscular impressions have not been 
satisfactorily observed. 
The largest specimens of this species are about half an inch long and 
five-eighths of an inch wide, while the depth in gibbous specimens is 
little less than the length. 
This shell has much the appearance of a broad gibbous Rhynchonella; but 
the perforated beak, false area and punctate structure, serve to distinguish it 
from that genus. I have referred this shell to Trematospira, from the generally 
similar character of the beak and perforation, and the punctate structure of the 
shell. I have not discovered the existence of spires ; but my specimens are too 
few to admit of the necessary examinations. 
Geological formation and localities. This fossil has been found near Bellona, 
Ontario county; near Geneseo and York, Livingston county; and at Pavilion and 
Darien in Genesee county, New-York. 
[ Palaeontology IV.] 
85 
