17© 
PALAEONTOLOGY 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 papillse. The cardinal process 
is strong at its base, and bilobed above. There are obscure evidences of 
teeth-sockets. 
This species resembles some of the forms 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. 
