LAMELLIBRANCHIA TA. 
201 
Anterior end short, acute. Wing large, not distinctly limited, extending 
almost to the posterior extremity; margin scarcely concave below; 
extremity produced, acute. 
Test marked by fine concentric striae, which are crowded into fascicles at 
nearly equal intervals, rounded upon the upper part of the body, and sub-angular 
on the lower part. The striae are crowded and lamellose on the anterior; 
on the posterior slope they make a short abrupt curve, passing over the wing 
with a gently forward direction and curving backward just below the hinge¬ 
line. In the weathered surface of the right valve the concentric undulations 
are stronger, the post-cardinal slope is marked by strong interrupted radii 
which appear to belong to the intimate structure of the shell. The hinge is 
marked by a single narrow groove. 
A specimen of the left valve has a length of 22 mm., height 14 mm., and 
hinge-line about 21 mm. A right valve occurring in the same association 
has a length, from beak to base, of 26 mm., height 18 mm., and hinge-line 
about 25 mm. 
This species is quite unlike any here described. 
Formation and locality. In the lower beds of the Chemung, group at the 
Inclined Plane, near Ithaca, N. Y. 
Leptodesma Cadmus, n. sp. 
PLATE XC, FIGS. 6, 7. 
Shell of medium size, rhomboidal; body oblique, narrowly ovate or sub- 
cylindrical; length nearly one-third greater than the height; ante-byssal 
margin somewhat vertically truncate, sloping below into the broadly curving 
base; posterior margin abruptly curved. 
Valves convex, gibbous above; the right valve smaller than the left. 
Hinge-line straight, nearly equal to the height of the shell. 
Beaks at about the anterior third of the hinge-line acute, directed forward, 
prominent, arching over the cardinal line. Umbonal region abruptly gibbous, 
subtending an acute angle. 
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