«60 PALAEONTOLOGY OF NEW- YORK. 



Young specimens of this species might be mistaken for the )'oung of P. galeatus, 

 which are, like this, destitute of plications. Its form, however, is much more elon- 

 gated, and the beak more extended. 



Fig. 2 a, b, c, d. Dorsal, profile and front views of young individuals. 



Fig. 2 t. Dorsal view of a specimen of medium size. 



Fig. 2 f, g. Front and profile views of the same specimen. 



Fig. 2 A, «". Profile and front views of a very gibbous specimen, which represents the cha- 

 racter of the ordinary full-grown individual. 



Fig. 2 k, I. Dorsal and profile views of an unusually large specimen from which the shell 

 is partially exfoliated, showing the two septa of the dorsal valve. 



Geological position and locality. In the Upper Pentamerus limestone of the Lower 

 Helderberg group; being the fossil which characterizes the higher compact beds of 

 the group, as P. galeatus does those of the lower beds of the same : Helderberg 

 mountains, Schoharie, Carlisle and other places. " 



Pentamerus veriieuili. 



Plate XL VIII. Fio. 1 a - y. 



Shell subglobose ; transverse diameter generally greater than the height. 

 Ventral valve nearly circular or transversely elliptical, more depressed 

 than the opposite, having a distinct sinus, commencing near the beak 

 and regularly widening and deepening to the front, where it terminates 

 in a short truncated extension fitting into a corresponding depression 

 in the front of the other valve : beak shorter than the opposite, per- 

 forated by a triangular or subcircular foramen, which is generally 

 covered by the strongly gibbous incurved beak of the other valve. 

 Dorsal valve very much elevated : beak extremely gibbous or ventri- 

 cose, and strongly incurved. 



Surface marked by from twenty-four to thirty sharply angular elevated 

 plications, which increase by interstitial addition and bifurcation : 

 from four to six of the plications on the ventral valve usually occupy 

 the sinus ; while from five to eight of. those on the dorsal valve are 

 very slightly elevated, so as to form a flat rather indistinct mesial fold. 



