LAMELLIBRANCHIATA. 193 



Leptodesma Matheri. 



PLATE -\Xn, FIGS. S, !), 10; AND J'LATli LXXXIX. flG. iS. 



Lfptndt.fiiin Mathiri, Hall. V-A. N. Y.. vol. v. jit. 1. Plates ami Exiilanations : PI. 22, fiffs. S-10. 



Jan.. 1S83. 



Shell of medium size, rhoiiiboidal ; body ovate, oblique at au angle of 45° with 

 the hiuge-liiie ; length about one-half greatei" than the height ; ante-byssal 

 margin vertical in front, joining the broad curvature of the basal margin ; 

 posterior margin produced and abruptly recurved. 



Left valve moderately convex below, scarcely gibbous aljove. Right valve 

 shorter and less convex. 



Hinge-line straight ; length much less than tlie length of the valve, Init 

 nearly equal to the height. 



Beaks at about the anterior third of the hinge, obtuse, directed slightly 

 forward, not very prominent. Uml)onal region moderately gibbous, sub- 

 tending an acute angle. 



Anterior end protluced, tiattened, acute at the extremit}-. \\n\g of 

 moderate dimensions, extending nearly to the posterior extremity ; margin 

 gently concave ; extremity produced and acute. 



Test thin, marked by fine concentric striae which are tasciculate on the 

 body of the shell, producing an undulated appearance. The stria? are 

 sub-lamellose on the anterioi- and regular over the wing. 



Interior unknown. 



A left valve has a length of o5 mm., height 23 mm., and hinge-line about 

 28 mm. A similar right valve has a length of 21) mm., and height 20 mm. 



This species differs from L. robustum in its comparatively larger wing, without 

 distinct sinus in the margin, and the more produced anterior end. It resem- 

 bles L. potens, but differs in its more produced anterior end, less gibbous body, 

 and more abruptly recurved posterior margin. 



Formation and locality. In the upper beds of the Chemung group, near 

 Bradford, McKean county. Pa. 

 25 



