384 PALEONTOLOGY OF NEW YORK. 



Valves moderately convex below and posteriorly, gibbous in the middle 

 and umbonal region. 



Beaks anterior, directed forward and strongly incurved. Umbonal slope 

 sub-angular, extending to the posterior margin above its greatest extension ; 

 above this there is another sub-angular fold. Post-cardinal area very wide. 

 Cincture obscure, often obsolete, continuing as an undefined depression from 

 the umbo to the basal margin near the posterior extremity. 



Surface marked by very fine, regular concentric striae, and upon the ante- 

 rior portion of the shell by angular concentric undulations, which become 

 obsolete on the posterior slope, usually between the cincture and the um- 

 bonal ridge ; these undulations are rarely duplicate in the upper and middle 

 portions of the shell. In well-preserved specimens extremely fine radiating 

 striae have been observed on the post-cardinal slope, which may also extend 

 to other portions of the shell. In many specimens there are a few continu- 

 ous, stronger radiating striae along the posterior slope, upon the umbonal 

 angle and above. Interior unknown. 



Four specimens measure respectively 25, 36, 38 and 39 mm. in length, 

 and 14, 20, 21 and 21 mm. in height. The largest individual observed has 

 a length of 55 mm. 



This species somewhat resembles G. precursor, but differs in the comparatively 

 finer concentric folds, smaller anterior end, more cimeate form, less defined 

 umbonal angle and the very oblique cincture. 



At some localities specimens are extremely abundant, usually of small size 

 and much distorted from pressure. In all the collections only two or three 

 individuals have been obtained preserving the entire form. 



Formation and localities. In the shales of the Hamilton group, in Otsego, 

 Chenango, Madison. Onondaga and Seneca counties, N. Y. 



