112 
PALAEONTOLOGY OF NEW YORK. 
Valves convex; the right valve less convex and smaller than the left valve. 
Hinge-line straight, less than the length of the valve. 
Beak acute, prominent, inclined forward, close to the anterior end of the 
shell. Umbonal region prominent, and in the left valve gibbous; subtending 
an acute angle. 
Ear small, separated from the valve by a broad sulcus, beyond which it is 
a mere fold in the shell. Wing large, triangular, flat, extending nearly to 
the margin of the valve; limited, in the left valve, by a more or less dis¬ 
tinct sulcus, and the abrupt bending of the concentric striae ; margin con¬ 
cave; extremity acute. This character of the wing is somewhat less marked 
in the right valve. 
Test thick; the left valve marked with strong, prominent, rounded radii, 
regularly alternating with finer ones on the posterior half of the valve; 
crossed at regular intervals by strong concentric lamellae; the interspaces 
marked by fine lines of growth. From maceration or exfoliation these 
surface characters are usually only partially preserved, or nearly obsolete. 
In certain conditions the rays become nodose where crossed by the lamellae, 
and in many examples the concentric striae interrupt the rays, leaving them 
alternating above and below the lamellae. (See pi. xviii, figs. 13, 14.) 
The surface characters of the body are continued on the wing, while on 
the ear the concentric striae are crowded and conspicuous, and the rays 
obsolete. In the right valve the markings are much subdued, the rays often 
obsolete, especially on the lower part of the valve, and the concentric lamellae 
are simple undulations of the surface. 
Ligamental area narrow, linear; marked, apparently, by a single groove. 
The cast shows a fold corresponding to the sulcus limiting the wing. Pallial 
line extending from just anterior and below the umbo to the posterior portion 
of the body and thence recurving, terminating in a small oval muscular impres¬ 
sion. No evidences of teeth are seen in the casts or the interior of valves. 
A large specimen has a length of 76 mm., height 56 mm., hinge-line 62 
mm. A medium sized example has a length of 52 mm., height 43 mm., 
hinge-line 42 mm. 
