218 
PALEONTOLOGY OF NEW YORK. 
This species resembles L. Orcus, but the wing is wider, less deeply and 
abruptly sinuous, and the striae turn more abruptly forward at the junction 
of the body and wing. The body is narrower and more oblique than in 
L. Lysander, and the anterior end less prolonged. 
Formation and locality. In the shales of the Upper Chemung group near 
Canton. Bradford county, Pa. 
Leptodesma alatum, n. sp. 
PLATE XC. FIGS. 26, 27. 
Shell of medium size, sub-rhomboidal; body ovate, rapidly narrowing toward 
the beak, oblique, making an angle of about 50° with the hinge-line ; length 
one-third greater than the height; ante-byssal margin oblique and curving into 
the distinct sinus ; ventral margin gently curved, joining the broad, rounded, 
posterior extremity. 
Left valve gently convex below, becoming more convex and slightly 
gibbous above the middle. Bight valve depressed in the lower portion, 
in the middle and above about equally convex with the left. 
Hinge-line straight, a little less than the length of the valve, and greater 
than the height. 
Beaks sub-anterior, directed forward, acute, but little elevated above the 
hinge-line in the left valve. Umbonal region narrow, and abruptly gibbous. 
In the right valve the beak is depressed, and the umbonal region a little 
less gibbous than in the other valve. 
Anterior end small, acute at the extremity, rounded below to the distinct 
byssal sinus. Wing large, wide-triangular, joining the body at the posterior 
extremity; margin moderately concave, the greatest concavity being above 
the middle, from which point it turns abruptly outward and the extremity 
is produced into a spiniform extension. In the right valve the wing is less 
distinctly limited from the body than in the left. 
Test thin, marked by sharp, elevated, concentric striae, with finer inter¬ 
mediate lines of growth. The stronger striae are somewhat regular upon the 
body and wing, while on the anterior side they are crowded and fasciculate. 
