LAMELLIBRANCHIATA. 
103 
flat interspaces wliich 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. 
Interior unknown. 
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. 
The large right valve represented in fig. 4, of plate xvi, is referred with doubt 
to this species on account of its greater size. This specimen shows a large, 
broad, elliptical muscular impression near the middle of the post-cardinal slope of 
the valve. 
j 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 reasons I have considered it desirable 
to distinguish them, for the present, under a sub-generic head. 
