LAMELLIBRANCHIATA. 103 



flat interspaces which are sometime.s marked by intermediate rays; crossed 

 by fine, concentric lines of growth. Tlie 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 lamellaj 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 repi-esented 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. 



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

 Elmira, an<l 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. 



