Lamellibranchiata. 293 



regularly rounded, extended on the post-basal portion and sloping 

 rapidly into the sinus of the wing. Left valve moderately convex. 

 Eight valve depressed-convex above, flat or concave below, appa- 

 rently smaller than the left. Hinge-line straight, longer than the 

 length of the valve, and extending beyond both margins. Beak 

 small, acute, directed forward, situated at about the anterior fifth of 

 the hinge. Umbonal region undefined, not prominent. Ear about 

 one-fifth the length of the wing, separated from the body of the 

 valve by a shallow, undefined sulcus and marked byssal sinus ; 

 margin convex ; extremity rounded. Wing large, triangular, ex- 

 tended, defined beyond only by the change in the curvature of its 

 margin from that of the body of the valve ; margin deeply con- 

 cave; extremity acute. 



Surface of left valve marked with regular, rounded, sub-equal rays 

 with flat interspaces which are sometimes marked by intermediate 

 rays; crossed by fine, concentric lines of growth. The cardinal 

 expansions of both valves show the same character of radii as the 

 body of the left valve. The right valve has subdued rays in the 

 lower posterior portion; the upper part shows only the concentric 

 markings in regular lamellae of growth. 



One specimen is 34 mm. in length, 28 mm. in height, hinge-line 

 44 mm. Another is 38 mm. in length, 32 mm., in height, hinge- 

 line about 47 mm. 



This species has a resemblance to the left valve of P. avis ; but the 

 left valve of the present species has greater convexity, more extended 

 hinge-line, and a broad, deep sinuosity in the margin of the wing. 



Formation and localities. In the Chemung group at Buck's quarry 

 near Elmira, and Chemung Upper Narrows, Chemung county, N. Y. 



The three following species have the right valves convex, and the 

 left valve nearly flat or concave, being the reverse in this respect of 

 the preceding species of the group. The hinge-area is narrow and 

 teeth have not been observed, while the surface markings are peculiar 

 and characteristic in the three forms here described. For these rea- 

 sons I have considered it desirable to distinguish them, for the 

 present, under a sub-generic head. 



