PRODUCTELLiE OF THE CHEMUNG GROUP. 
185 
The occlusor muscular impressions are well defined, semielliptical 
and of moderate breadth, appearing to be duplicate; the surface imme¬ 
diately below them is transversely corrugated over a small oval space. 
The cardinal muscular impressions are wide and comparatively short, 
flabelliform and deeply striated. The cast of the rostral cavity is grooved 
on the upper side, and its extremity simple; showing on each side, be¬ 
neath the apex, cavities made by the removal of the dental lamellae, 
while the cardinal line presents no evidence of an area beyond the thick¬ 
ness of the shell. 
The interior of the dorsal valve is papillose above, becoming papillose- 
striate towards the margin. There is a slender mesial septum, which 
becomes stronger above, and terminates in a short cardinal process : on 
each side of this are slightly oblique socket-plates, and between these 
and the hinge-line are narrow teeth-sockets. The cardinal process is 
distinctly bilobed, and each division is slightly divided or grooved at 
the extremity. The occlusor muscular impressions are small oval-ovate 
spots, but are not well-preserved in any specimen seen, yet the reniform 
vascular impressions are sometimes strongly marked. In addition to the 
ordinary vascular impression, which terminates abruptly at some dis¬ 
tance from the median septum, there is sometimes another mark of 
similar character beginning at a little distance within the termination 
of the reniform impression, and curving forward to the termination of 
the median septum, and thence recurving it terminates on the other 
side in the same manner as it began, describing part of an elliptic curve. 
This shell has a very symmetrical form, and is the largest among the species 
of this subgenus. It is readily distinguished from all others by its regular wide 
form and equal convexity, as well as by its distantly disposed and very slender 
spines, by the peculiar punctate structure of the exfoliated shell, and the striato- 
punctate surface of the cast. 
The form and chai’acter of well-preserved" specimens are shown in figures 36, 37 and 38, 
which give the outline, cardinal and profile views of a ventral valve. Figures 39 and 40 
are cardinal views of the casts of ventral valves, showing the muscular impressions, and 
one of them showing on eaqh side of the beak the cavities made by the dental lamellae. 
Figures 30 and 31 are of the interior of two dorsal valves ; and figure 32 is a cardinal 
view, showing the contour of the interior. 
Geological formation and localities. This species occurs in the arenaceous and 
semicalcareous beds at Conewango, at Napoli, and near New-Albion, Cattaraugus 
county. 
[ Palaeontology IV.] 
24 
