TREMATOSPIRA OF THE HAMILTON GROUP. 373 



DoBSAL 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. 



Thi8 shell has inuch tlie appearance of a broad gibbous Rhynchonella; but 

 the perforated l>cak, false area and punctate structure, serve to distinguish it 

 from that genus. I have referred this ehell to TKEMAXOsrraA, from the generally 

 similar character of the beak and perforation, and the pimctate 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 Geueseo and York, Livingston county; and at Pavilion and 

 Darien in Genesee county. New- York. 



[Palaeontology IV.] 85 



