LAMELLIBRANCHIA TA. 
69 
Beak acute, well defined, arching over the hinge, directed forward, situated 
anterior to the middle of the shell and of the hinge-line. 
Anterior ear small, triangular, with a longitudinal fold; limited by a broad 
sulcus; margin slightly convex; extremity acute. Posterior ear large, broad- 
triangular, undefined; margin slightly concave, extending at right angles to 
the hinge, to the post-basal side of the valve; .extremity angular. Byssal 
sinus broad in the left valve. 
Test thin, marked by fine, alternating, filiform, radiating strise, which 
increase by implantation until they are very numerous at the basal margins 
of some specimens, crossed by fine lines of growth with a few concentric 
undulations. The ears preserve the same surface characters in a subdued 
degree. Many of the specimens are exfoliated and the surface markings are 
very obscure or obsolete. 
The characters of the interior have not been observed. 
A specimen of the left valve has a length of 14 mm., height 12 mm., hinge¬ 
line 16 mm. A larger example has a length of 19 mm., height 16 mm., and 
hinge-line 21 mm. 
This species was arranged with P. conspectus. The recent study and compari¬ 
son of a more numerous series of specimens shows several important differences, 
and necessitates the removal of this form from that species. 
The present species is more oblique, narrower, and the posterior ear is 
comparatively longer than in authentic forms of P. conspectus. 
In P. Hermes the hinge-line is more extended posteriorly, and the umbonal 
angle more obtuse, while in P. regularis the hinge is shorter, the extremity of 
the posterior ear rounded, and the margin continuous with the curvature of 
the pallial margin of the valve. 
Formation and localities. In the shales of the Hamilton group at Ludlowville, 
Tompkins county, and on the shores of Cayuga and Canandaigua lakes, N. Y. 
