LAMELLIBRANCHIATA. 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 striae, 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 obsciu-e 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 IC) nun. A larger example has a length of 19 mm., height 16 mm., and 

 hinge-line 21 mm. 



This species was arranged with P. conspechis. The recent study and compari- 

 son of a more numerous series of specimens shows several important dift'erences, 

 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. 



