222 
PALEONTOLOGY OF NEW YORK. 
A left valve has a length of 51 mm., height 37 mm., hinge-line about 
42 mm. 
Specimens of this species usually present a very rough and irregular aspect 
from the character of the shell, with its strongly lamellose structure and the 
coarse matrix in which it is usually found. The specimen figured is a cast of 
the interior, and is distinguished by the broad posterior end and abrupt narrowing 
toward the front, the obtusely sub-angular umbo, and the large anterior end. 
In a crushed specimen retaining both valves, the right valve is almost equally 
convex with the left; the surface is less lamellose and presents only gentle 
undulations. 
Formation and locality. In a coarse sandstone of the upper part of the 
Chemung group, on the road from Olean, N. Y., to Smethport, Pa. 
Leptodesma Biton, n. sp. 
PLATE XCI, FIG. 1. 
Shell large, sub-rhomboidal; body elongate-ovate, making an angle of about 
45° with the hinge-line ; height nearly two-thirds of the length ; ante-byssal 
margin oblique, slightly curving into a long, shallow sinus, which impresses 
the margin for more than half the length of the valve; basal and posterior 
margins broadly rounded. 
Left valve regularly and gently convex in the lower part, becoming more 
convex and gibbous above the middle. Right valve unknown. 
Hinge-line straight, greater than the height of the valve. 
Beak sub-anterior, directed forward, but little elevated above the hinge¬ 
line. Umbonal region abruptly gibbous, subtending an acute angle. 
Anterior end produced, narrow, acute at the extremity, limited by an 
oblique, shallow byssal depression. Wing large, joining the body at the 
posterior end, not distinctly separated ; margin slightly oblique below, gently 
concave above the middle, and curving slightly outward just below the car¬ 
dinal line. 
Test thin; cast marked by fine concentric striae of growth, which, at 
