ir« PALEONTOLOGY OF NEW- YORK. 



Dorsal valve moderately or deeply concave, and more abruptly curving 



to the front. 

 Surface of ventral valve marked by fine concentric striae, and, on the 

 ears, by a few short wrinkles which become obsolete on the sides of 

 the umbo. There are marks of four or five spines on each of the ears 

 just below the hinge-line, and a row of tubercles or spine-bases along 

 the summit of the wrinkles as far as they extend. The body of the 

 shell is covered with as many as twenty-five concentric rows of spi- 

 niferous tubercles from beak to base : these are closely arranged, and 

 only become a little irregular towards the front of the shell. The sur- 

 face of the cast is minutely punctate. 



The interior of the dorsal valve is strongly wrinkled on the hinge- 

 margin ; and these wrinkles are indistinctly continued across the valve, 

 and studded with numerous concentric ranges of tubercles, which indi- 

 cate the place of fossets or of spines on the opposite side : the interme- 

 diate space is marked by distantly disposed papillas. The cardinal process 

 is strong at its base, and bilobed above. There are obscure evidences of 

 teeth-sockets. 



This species resembles some of the forma of P. lachrymosa ; but the spiniferous 

 tubercles are smaller, more closely arranged and more numerous, while the umbo 

 of the ventral valve is narrower and somewhat abruptly attenuate. The dorsal 

 valve is studded with numerous tubercles on the interior surface ; while in that 

 species the surface is papillose, with indistinct wrinkles and some distant eleva- 

 tions. In P. lachrymosa, the divisions of the cardinal process are more divergent, 

 and each division is again bilobed ; while in this the divisions appear to be 

 simple. 



The species was originally described from a well-formed ventral valve ; the same 

 fragment of stone containing a dorsal valve of corresponding character. Numerous 

 individuals among later collections preserve a similar form and expression of 

 ventral valve, but none of them are quite so extremely arcuate. This feature, 

 however, is probably characteristic of the older shells, while the younger ones 

 would acquire this form from continued growth. A careful examination of all the 

 specimens from different localities has shown constant and reliable characters for 

 the determination of the species. 



The illustrations show the prevailing differences of form and character among 

 the specimens of this species. , 



