LAMELLIBRANCHIATA. 191 



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 posterior 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 Ijetween 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. 



Interior unknown. 



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 al)out 34 mm. 



This species bears some resemblance to L. robuslum, 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 symmetrically concave on the 

 margin. The form of the shell is very similar to L. longispinum, but difters 

 by its larger wing, the broader sinus and more regularly rounded posterior 

 extremity, and in the latter respect it resembles L. Shumardi, while it is 

 otherwise very different. 



Formation and localities. In the Chemung group at Portville and Little 

 Genesee, Alleghany county, N. Y. 



