384 
PALAEONTOLOGY OF NEW YORK. 
Valves moderately convex below and posteriorly, gibbons 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 line, 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 cuneate 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. 
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