290 Thirty-fifth Report ok the State Museum. 



and the corresponding portion of the posterior margin is gently 

 concave. Left valve moderately convex above, depressed-convex in 

 the lower part. Right valve depressed-convex toward the beak, flat 

 or slightly concave below. Hinge-line straight, central; length 

 nearly or quite equal to the length of the shell. Beak anterior, 

 acute, directed forward, arching a little over the hinge-line. Um- 

 bonal region not strongly defined, most prominent a little below the 

 hinge; subtending an acute angle. Ear short, nearly equilateral, 

 limited by a rounded sulcus ; margin concave, with a marked sinu- 

 osity and arching of the shell for the byssal sinus. Wing large, tri- 

 angular, and extending beyond the posterior margin of the valve, 

 not distinctly defined by a sulcus or by marked change in the sur- 

 face characters from the body of the shell ; margin straight or 

 slightly concave ; extremity obtuse. In the right valve the wing is 

 less distinctly defined than in the left. 



Test somewhat thin; left valve marked by slender, filiform rays 

 variously 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 inter- 

 spaces 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-lme 

 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. consimilis, differing in the propor- 

 tionally shorter hinge-line, greater height of body, and less obliquity. 



