LOWER HELDERBERG ROCKS. 
SB 1 
Trematospira simplex (n. s.). 
Plats XXYIII A. Fig. 2 a - /. 
Shell transversely elliptical or subrhomboidal, nearly once and a half as 
wide as long : hinge line scarcely declining from the beaks; cardinal 
extremities rounded : valves moderately and almost equally convex, 
closely compressed at the latero-cardinal margins. Yentral valve having 
a well defined mesial sinus, which extends more than half way from 
beak to base : beak elevated, scarcely incurved, and vertically trun¬ 
cated by a small perforation, between which and the hinge line is a 
well-defined deltidium. Dorsal valve moderately convex in the middle, 
and slightly elevated towards the front of the shell : beak closely 
incurved beneath the deltidium of the opposite valve. 
Surface marked by four or five simple plications on each side of the 
centre, two of which on the ventral valve are involved in the sinus; 
and three upon the opposite valve, the central one being broader and 
flattened towards the base : entire surface granulose or punctate, with 
faint indications of growth lines. 
Trematospira simplex, var. 
A larger imperfect specimen, apparently of the same species, has the 
beak slightly more incurved, the mesial plication depressed in the centre, 
and those on each side bifurcating near the base, as are those which 
border the sinus of the ventral valve. The mesial sinus in this specimen 
is much deeper, and the front of the opposite valve much more elevated. 
This species, in its general form, resembles the T. camura of the Niagara group; 
but the shell is somewhat more robust, and the plications are less numerous and 
stronger. The Niagara species has two smaller plications forming the mesial fold, 
which often become obsolete before reaching the beak; and the sinus of the ventral 
valve is often marked by one or two smaller plications, which sometimes extend 
more than half way to the beak. It differs also from this species in the strong lines 
of growth, which are seen in many of the specimens. These characters are shown 
in the figures 3 k , Z, m, n of Plate lvi, Vol. ii, Paleontology of New-York. 
