LAMELLIBRANCHIATA. 99 



alternating and bifurcating, or sometimes simple from the uml)0 to the liase, 

 continuing over the wing, and somewhat stronger on the anterior cardinal 

 extremity than on the posterior ; the interspaces flat, nuich wider than the 

 rays. The surface is crossed by fine, closely arranged, concentric stria?, 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 strife 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 lieight, 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 tlie l)roadly rounded base and interrupted rays, which are 

 coarse and fine, and irregularly arranged. 



•This species reseml)les P. rigida, but the radii lia\e 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 k'ss olilicpiity. 



Formation and locality. In the Chemung group at Chemung Narrows, Che- 

 mung county, N. Y. 



