Lamellibeanchiata. 393 



with L. Phaon and L. Lichas, which are among the extreme variations 

 of the species of Leptodesma. 



Formation and locality. In the Upper Chemung group, Lindley 

 township. Steuben county, N". Y. 



Section /, mytiloidea. 

 Leptodesma acutieosteum. 



Leptodesma aentirostrum, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



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. Left 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-line. Umbonal region narrow and moderately 

 gibbous, subtending a very acute angle. The byssal depression 

 reaches the margin just below the beak, leaving a very small, acute, 

 anterior extremity. Wing large, undefined, joining the body at the 

 posterior end ; margin oblique, essentially straight, not recurved at 

 the cardinal extremity. 



Test thin, leaving upon the cast the marks of the striae of growth 

 which are more or less fasciculate upon the body, and crowded and 

 lamellose in front. Hinge furnished with a distinct posterior tooth. 

 Interior unknown. 



The specimen described has a length of 32 mm., height 24 mm., 

 and hinge-line 22 mm. 



This species somewhat resembles L. Or odes, but the beak is more 

 nearly anterior, the anterior end much smaller and more acute, the 

 byssal sinus not extending so low on the body of the valve, and the 

 body of the shell more distinctly attenuate toward the beak. It is 

 especially distinguished by its narrow slender beak and small anterior 

 end. 



Formation and locality. In a sandstone of the Upper Chemung 

 group at Warren, Pa. 



[Sen. Doc. No. 38.] 50 



