PALAEONTOLOGY OF NEW-YORK. 
and lateral margins. Hinge line equal to the greatest width of the 
shell, crenulated. Area linear, vertically striated. Foramen small, tri¬ 
angular, closed in full-grown individuals. 
Surface marked by fine obscure closely arranged radiating striae, crossed 
on the depressed part of the valves by small regular concentric 
wrinkles. 
The interior of the ventral valve shows a narrow almost linear area, the inner 
margin of which is crenulated, the crenulations extending on each side more than 
half way from the centre to the cardinal extremities. In old shells there is no evi¬ 
dence of foramen, but a slight depression in the inner edge of the area (which may 
be accidental), with the two cavities beneath for the reception of the processes of 
the opposite valve. The vascular area is somewhat broadly ovate or cordate, with a 
prominent imprint of the adductor muscles, from which extends a narrow ridge 
through the centre of the muscular area. Another individual, apparently identical 
with this one, shows the muscular area more distinctly divided through the centre. 
An impression of the interior of the dorsal valve shows the strong imprints of 
adductor muscles, wdtli cavities made by the teeth or cardinal processes. 
This shell bears a general resemblance to Strophomena depressa. It differs, how- 
ever, remarkably from that species in its resupinate character, the convexity and 
concavity of the valves being in the opposite direction : the area of the ventral valve 
is also wider, and the cardinal margin is crenulated. 
PLATE XXI. 
Fig. 5 a. The dorsal valve, showing obscurely the concentric wrinkles and the punctate 
surface where the shell is exfoliated. 
Fig. 6 a. Interior of the ventral valve. 
Fig. 6 b. Profile of the preceding. 
Fig. 6 c. Enlargement of the crenulations of the hinge line of 6 a. 
Fig. 7 a. Interior of a ventral valve. 
Fig. 7 b. Profile of the same. 
PLATE XXIII. 
Fig. 1 «, b, c. Ventral, dorsal and profile views of an entire specimen. 
Fig. 2 a, b, c. Dorsal and profile views of a full-grown individual. 
Fig. 2 e. Enlargement of the fiAie equal striae. 
Fig. 3. Impression or cast of the interior of the dorsal valve. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains. 
