LAMELLIBRANCHIATA. 229 



Wing large, undefined ; margin very oblique in the lower part, slightly 

 concave above, the greatest concavity being a little below the hinge-margin ; 

 extremity produced into an angular extension. The right valve shows less 

 distinction between the body of the valve and the wing. 



Test thin, marked by distant lamellose lines of growth with intermediate 

 finer stria 1 . These lines are more regular upon the wing and very much 

 crowded and fasciculate upon the anterior side of the body. 



Interior unknown. 



Hinge, posterior to the beak, furnished with a slender lateral tooth 



A left valve of this species has a length of 55 mm., height 35 mm., hinge- 

 line 37 mm., and the body, at the posterior end, having a width of 25 mm. 

 A right valve has a length of 45 mm., height 30 mm., and hinge-line about 

 28 mm. 



The specimens of this species observed are casts of the interior in coarse 

 sandstone, or impressions of the exterior in the same rock. The species is 

 distinguished by its straight ventral side and the nearly parallel direction of 

 the sides of the body in the left valve. 



Formation and localities. In a coarse sandstone of the Upper Chemung group, 

 on the road from Olean, N. Y., to Smethport, Pa., and at Bradford, Pa. 



Leptodesma arciforme, n. sp. 



PLATE XCI, FIG. 12. 



Shell large, narrow, sub-rhomboidal ; body sub-cylindrical, oblique at an angle 

 of about 30° to the hinge-line ; length nearly one-half greater than the 

 height ; anterior margin oblique, merging into the broad, low curvature of 

 the ventral margin ; posterior margin abruptly rounded and curving into the 

 undefined wing. 



Valves very convex, strongly gibbous, obtusely sub-angular along the 

 middle ; the greatest convexity distant from the beak one-third the height 

 of the valve. The right valve is less gibbous, but otherwise similar to the 

 left. 



