LAMELLIBRANCHIA TA. 
99 
alternating and bifurcating, or sometimes simple from the umbo to the base, 
continuing over the wing, and somewhat stronger on the anterior cardinal 
extremity than on the posterior; the interspaces flat, much wider than the 
rays. The surface is crossed by fine, closely arranged, concentric striae, which 
are occasionally crowded, giving an unequal elevation to the rays. These are 
also often interrupted and undulated, either from accident to the shell or 
intermittent growth, and are sometimes nearly obsolete at the base. The 
right valve is marked by the same interrupted rays on the wing; on the 
body of the shell the radii are obsolete, but the concentric striae are preserved. 
The specimens are generally found as casts, or in an extremely macerated 
condition. 
Ligamental area wide, marked by fine, narrow grooves the whole length of- 
the hinge. Obscure indications of lateral teeth have been seen. 
A left valve is 62 mm. in length, 76 mm. in height, hinge-line 55 mm. 
Another one has a length of 55 mm., height and hinge-line each 62 mm. 
A very large specimen is 85 mm. in height, 102 mm. from beak to post- 
basal margin, length 70 mm., hinge-line about 65 mm. A right valve has a 
length of 55 mm., height 60 mm., hinge-line about 60 mm. 
The form varies from nearly erect to considerably oblique, and is usually 
characterized by the broadly rounded base and interrupted rays, which are 
coarse and fine, and irregularly arranged. 
This species resembles P. rigida, but the radii have an appearance of less 
rigidity, the hinge-area is narrower and the shell is more oblique. It is closely 
allied to P. comimilis , differing in the proportionally shorter hinge-line, greater 
height of body, and less obliquity. 
Formation and locality. In the Chemung group at Chemung Narrows, Che¬ 
mung county, N. Y. 
