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. 



Leptodesm \ u.ati-m. n. sp. 



1'I.A'l I. \i FIGS 26, .'7 



Shell of medium size, sub-rhomboidal ; body ovate, rapidly narrowing toward 

 the beak, oblique, making an angle of about -30° 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. Right 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, hut 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 

 distinctlj limited from the body than in the left. 



Tesl thin, marked hy sharp, elevated, concentric stria', with liner 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. 



