Lamellibranchiata. 389 



wing and very much crowded and fasciculate upon the anterior side 

 of the body. 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. 



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 Unpublished. 



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 ab- 

 ruptly rounded and curving into the undefined wing. Valves very 

 convex, strongly gibbous, obtuse'ly 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. Hinge-line straight ; length about equal to the height of 

 the shell. Beaks prominent, directed forward and arching over the 

 hinge, situated at about the anterior third of the cardinal line. 

 Umbonal region angularly gibbous above, abruptly defined along 

 the posterior slope. Umbonal angle acute. Anterior end short, 

 angular. Wing an undefined, narrow-triangular expansion ; mar- 

 gin straight or a little concave ; extremity very slightly produced. 



Test moderately thick, marked by fine concentric striae, which are 

 fasciculate on the body of the shell, and crowded and lamellose over 

 the anterior. 



A large left valve has a length of 60 mm., height 36 mm., and 

 hinge-line 37 mm. 



This species is distingushed from L. Lichas by its larger and 

 stronger form, being very gibbous along the entire length of the shell 

 and with a proportionally smaller wing. 



Formation and locality. In the upper part of the Chemung group 

 at Mansfield, Tioga county, Pa. 



