192 
PALAEONTOLOGY OF NEW YORK. 
Leptodesma Billingsi, n. sp. 
Shell large, sub-rhomboidal; body elongate-ovate, or sub-cylindrical, very 
oblique; length about twice the height; ante-byssal margin rounded, 
becoming straight or slightly concave at the sinus; ventral margin broadly 
curving; posterior margin extended, acutely recurved at the post-basal 
extremity. 
Left valve gibbous on the umbo, convex below. Right valve unknown. 
Hinge-line straight; length, including the spine, nearly equal to the length 
of the valve. 
Beak sub-anterior, obtuse, slightly oblique, prominent. Umbonal region 
somewhat gibbous, subtending an acute angle. 
Anterior end limited by a shallow byssal depression, acute and nasute at 
the extremity. Wing long, narrow-triangular, extending below the middle 
of the valve; margin sinuate; extremity produced into a spine. 
Test thin, marked by fine concentric striae which are abruptly arched on 
the posterior slope, regular upon the wing, and crowded into fine irregular 
fascicles which are conspicuous on the anterior. Some specimens show sharp, 
elevated, lamellose striae at equal distances on the surface of the body. The 
surface characters vary with the different states of exfoliation and maceration. 
The largest specimen observed has a greatest length, from beak to base, 
of 57 mm., height 36 mm., hinge-line, to base of spine, 35 mm. A small 
example of the left valve has a length of 34 mm., height 18 mm., and hinge¬ 
line about 35 mm. 
This species is more elongate and oblique than L. longispinum , the wing extends 
farther down along the posterior slope, and the anterior extremity is fuller and 
more acute. 
Formation and localities. In the Chemung group at Panama, N. Y. Other 
specimens from near Elmira, N. Y., and Mansfield, Tioga county, Pa., are 
referred to this species with reservation. 
