160 PALEOZOIC PALEONTOLOGY. 
angular, radiating plications on each side of the fold and sinus, with 
one plication in the bottom of the sinus of the pedicle valve and two 
upon the fold of the brachial valve. The interlocking, angular plica- 
tions give to the anterior and lateral margins of the shell, in an 
anterior view, a sharply zig-zag outline. Crossing the plications, and 
parallel with the margin, are fine, concentric lines of growth, which 
can only be detected by the aid of a magnifying glass. Pedicle valve 
strongly arched from beak to front, the plications bounding the sinus 
very prominent, the lateral slopes concave, so that the valve just at 
the beak and a little in front is subcarinate. Sinus not continuous. 
to the beak. At the beak the median plication is slightly elevated 
above the two adjacent ones, but in passing anteriorly it soon becomes. 
depressed below them, and at the anterior margin occupies the bottom 
of the rather profound sinus. Brachial valve less convex than the 
opposite one, flattened in the middle and passing in a convex curve 
to the margin all around, except directly in front in the area oceu- 
pied by the median fold. The fold, starting at the beak as a narrow, 
shallow sinus, becomes elevated above the general surface of the shell 
at a point about one-third the distance from the beak to the anterior 
margin. 
The dimensions of a perfect specimen are: length, 5 mm.; width, 
6 mm.; thickness, 3 mm. 
Remarks.—This species was originally described from the Trenton 
limestone of Lewis county, New York, but it seems to be a rare shell 
in this formation. 'The shell usually representing the species in col- 
lections is from the upper Cincinnatian beds of Southern Ohio and 
Indiana, and has been well illustrated in the Ohio Paleontology, 
Volume I. In the New Jersey collections the species is represented 
by a single, perfect individual. It is much smaller than the specimens 
from Indiana and Ohio, being only 5 mm. in length and 6 mm. wide. 
In general form it agrees more closely with the figures of the typical 
form of the species as illustrated in the New York Paleontology, 
but is even smaller than the specimens there figured. In the concave, 
lateral slopes of its pedicle valve and the subcarinate character of 
the posterior portion of the same valve it is quite different from the 
Ohio and Indiana shells, and it is quite possible that the Cincinnatian 
species should be considered as distinct from the Trenton limestone 
specimens. 
