188 
PALAEONTOLOGY OF NEW YORK. 
A specimen of a left valve has a length of 10 mm., height 7.5 mm., hinge¬ 
line about 10 mm, A similar specimen has a length of 11 mm., height 6.5 
mm., and hinge-line about 11 mm. A right valve has a length of 9 mm., 
height 6 mm., hinge-line about 8 mm. Another one has a length of 16 mm., 
height 11 mm., hinge-line about 16 mm. 
This is a small and abundant species, often occurring in great numbers on 
the surfaces of shaly layers. It is distinguished by its form and the regular 
lamellose appearance of the surface. 
Formation and localities. In the shales of the Chemung group at Connewango, 
Cattaraugus county; Nanticoke Springs, Broome county, and south of Ithaca, 
N. Y. 
Leptodesma potens. 
PLATE XXI, FIGS. 21, 30; PLATE XXII, FIGS. 11,12,19, (20?) 21; and PLATE LXXXIX, FIG. 7. 
Leptodesma potens, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PL 21, figs. 21, 30; pi. 22, 
figs. 11, 12, 19, 21. Jan., 1883. 
Shell large, sub-rhomboidal; body ovate, oblique, broad below; length more 
than one-third greater than the height; ante-byssal margin sub-truncate, 
erect, oblique below; basal margin broadly curving; posterior margin broad, 
extended and abruptly recurving at the upper extremity. 
Left valve convex, gibbous above; the greatest convexity at one-third the 
height of the shell from the beak. Right valve almost equally convex, more 
abruptly gibbous on the umbo, and comparatively shorter and wider. 
Hinge-line straight, about equal to the length of the shell. 
Beak sub-anterior, obtuse, erect, prominent, arching over the hinge. 
Umbonal region gibbous, very prominent, limited anteriorly by a shallow 
depression extending from a point anterior to the beak to the margin at one- 
third the height of the shell below the hinge-line; posteriorly limited by the 
abrupt post-cardinal slope. Umbonal angle acute. 
Anterior extremity extended and angular. Wing large, triangular, joining 
the body near the posterior extremity, distinctly limited; margin broadly 
