234 PALEONTOLOGY OF NEW YORK. 



Anterior end convex. Limited by a shallow, oblique, byssal depression which 

 is nearly as long as the height of the valve; length one-half the height; 

 extremity slightly produced. Wing triangular, scarcely defined, joining the 

 body at the posterior extremity ; margin oblique, straight or slightly convex ; 

 extremity obtuse-angular. An imperfect right valve presents about the same 

 characters of wing. 



Test marked by fine striae of growth which are strongly fasciculate on the 

 anterior end. 



Interior unknown. 



A left valve has a greatest length of 31 mm., height 20 mm., and hinge- 

 line 21 mm. 



This species is distinguished by its short, broad, sub-quadrate form. It has 

 very much the aspect of Mopiomorpha, but clearly belongs with L. Phaon and 

 L. Lichas, which are among the extreme variations of the species of Lepto- 



DESMA. 



Formation and locality. In the Upper Chemung group, Lindley township, 

 Steuben county, N. Y. 



Section /, mytiloidea. 

 Leptopesma acutirostrum, n. sp. 



PLATE XCI, FIG. 21. 



Shell of medium size, rhomboid-ovate ; body narrowly ovate and rapidly 

 attenuate toward the beak, oblique, making an angle of a little less than 

 45° with the hinge-line; length nearly one-third greater than the height ;. 

 ante-byssal margin oblique, sloping into the shallow sinus; broadly rounded 

 below, the curvature continued into the posterior extremity. 



Lett valve depressed-convex in the lower part, moderately convex in the 

 middle, and slightly gibbous above. Right valve unknown. 



Hinge-line straight, less than the height of the valve. 



Beak anterior, acute, directed forward, scarcely rising above the hinge- 



