218 PALEONTOLOGY OF NEW YORK. 



This species resembles L. Orcus, but the wing is wider, less deeply aud 

 iil)niptly sinuous, and the striae turn more aljruptly forward at the junction 

 ol' the bod}' ami winy. 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. 



Shell of medium size, sul)-rlionil)()iilal : hdily ovate, rapidly narrowing toward 

 the beak, oblique, making an angle of about 50° with the hinge-line ; length 

 one-third gi-eater than the height ; ante-byssal margin oblique and <ur\ing into 

 the distinct sinus ; ventral margin gently cm'ved, joining the broad, rounded, 

 posterior extremity. 



Left valve gently convex below, becoming more couA-ex and slightly 

 gibbous above tlic middle. Right valve depressed in the lower portion, 

 in the middle and abcne 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, l>ut little elevated al)0\e the 

 hinge-line in the left valve. Umbonal region narrow, and abruptly gibbous. 

 In the right \ alve the beak is depressed, aud 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 moderateh' 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 b}^ 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. 



