PALAEONTOLOGY OF NEW-YORK. 
Young specimens of this species might be mistaken for the young 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 «, b, c, d. Dorsal, profile and front views of young individuals. 
Fig. 2 e. Dorsal view of a specimen of medium size. 
Fig. 2 f, g. Front and profile views of the same specimen. 
Fig, 2 h, i. Profile and front views of a very gibbous specimen, which represents the cha¬ 
racter of the ordinary full-grown individual. 
Fig. 2 k, l. 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 yemeuili. 
Plate XLYIII. Fig. 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. 
