LOWER HELDERBERG ROCKS. 
257 
Pentamerus galeatus. 
Plate XLYI. Fig. 1 a - z ; and Plate XLYII. Fig. 1 a - m . 
Shell varying from ovoid to subglobose and transversely elliptical. Ven¬ 
tral valve gibbous, becoming in old shells very ventricose in the um- 
bonial region ; a strong mesial fold along the centre of the lower half 
of the valve : beak ventricose, arched, and strongly incurved over 
that of the opposite valve. Dorsal valve often nearly circular or trans¬ 
versely elliptical, gibbous above; area with or without a defined mesial 
sinus towards the lower margin : beak incurved, and filling the tri¬ 
angular foramen beneath the beak of the ventral valve. 
Surface, in extremely young shells, smooth, or marked only by concen¬ 
tric lines of growth; in older forms, having longitudinal plications 
more or less developed, or rarely with none. Old shells variously pli¬ 
cated ; the plications simple or bifurcating, and crossed by fine con¬ 
centric striae of growth, which sometimes become stronger imbricating 
laminae towards the margin of the shell. 
Internally the dental lamellae are developed into a long spoon-shaped 
cavity or chamber, which forms a continuation from the triangular 
foramen or pit beneath the beak of the ventral valve. A central sep¬ 
tum extends from the beak of the ventral valve, half the length of the 
shell, and, in its upper part, is united to the conjoined dental lamellae 
or Y-shaped chamber of this valve. In the dorsal valve the two septa 
extending from either side of the beak are attached to the shell for 
about half the distance to the base, below which point they become 
free. 
This widely distributed species is characteristic of the compact limestones at the 
base of the Lower Helderberg group, and is known to occur in the same position 
as far south as Tennessee. 
It presents a great variety of form and surface markings. In New-York, the ex¬ 
tremely young specimens are always smooth, so far as I have observed. In the va¬ 
rious stages of growth, they exhibit every possible degree of development in the 
plications ; some individuals of nearly full size remaining smooth, while others are 
[ Paleontology III.] 33 
