360 Thirty-fifth Report ok the State Museum. 



margin extended and abruptly recurved. Left valve convex, gibbous 

 above. Right valve depressed-convex below, and abruptly gibbous 

 at the umbo. Hinge-line straight ; length less than the length of 

 the shell. Beaks sub-anterior, obtuse, erect, prominent, arching 

 over the cardinal line. Umbonal region gibbous and gradually 

 sloping down to the general convexity, abruptly limited on the pos- 

 terior side, subtending an acute angle. Anterior end extended, acute 

 and nasute. Wing large, triangular, slightly convex, joining the 

 body of the shell more than two-thirds of the length below the 

 beak ; margin symmetrically concave ; extremity produced and very 

 acute. In the right valve the anterior end is broad and flat ; the 

 wing is large, with no defined limit between it and the body of the 

 valve. 



Surface marked by elevated concentric striae which, at irregular 

 intervals, are fasciculate, producing undulations on the surface. 

 This character is often marked on the front of the shell where the 

 fascicles are composed of three or four distinct striae. 



A specimen of the left valve has a length, measured from beak to 

 base, of 44 mm., height 30 mm., hinge-line about 38 mm. Another 

 example has a length of 45 mm., height 25 mm., hinge-line 32 mm. 

 A right valve measures 43 mm. in extreme length, height 30 mm., 

 hinge-line about 34 mm. 



This species bears some resemblance to L. rohustum, but the body 

 is more oblique and not so broad below ; the anterior extremity is 

 acute ; the wing larger, more extended along the valve and very sym- 

 metrically concave on the margin. The form of the shell is very simi- 

 lar to L. longispinum, but differs by its larger wing, the broader sinus 

 and more regularly rounded posterior extremity, and in the latter re- 

 spect it resembles L. Shumardi, while it is otherwise very different. 



Formation and localities. In the Chemung group at Portville, Cat- 

 taraugus county, and Little Genesee, Alleghany county, N. Y. 



Leptodesma Billingsi. 



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 Unpublished. 



Shell large, sub-rhomboidal ; body elongate-ovate, or sub-cylindrical, 

 very oblique; length about twice the height; ante-byssal margin 

 rounded, becoming straight or slightly concave at the sinus; ventral 

 margin broadly curving ; posterior margin extended, acutely re- 

 curved at the post-basal extremity. Left valve gibbous on the umbo, 



