236 PALJEONTOLOGY OF NEW YORK. 



Left valve regularly convex in the lower half, becoming contracted, gib- 

 bous and sub-angular al)ove the middle. The right valve is smaller, the 

 liinge-line proportionally longer, the base of the valve and the wing much 

 more depressed, and the umbonal region narrow and more angular. 



Hinge-line straight, about equal to the height of the valve. 



Beaks sub-anterior, directed luiwanl, acute, scarcely raised above the 

 hinge-liiu'. Umbonal region iianow and gibbous, subtending a very acute 



angle. 



Anterior end short, aljniptly attenuate, acute, limited by a shallow byssal 

 depression which extends for hidf the length of the valve. Wing large, not 

 strongly limited, extending nearly to the posterior extremity of the body ; 

 margin very oblique, not concave, nor recurved below the cardinal line ; 

 extremity obtuse-angular. 



Test thin, marked by concentric stria3 which have left, their impression 

 upon the cast of the interior, showing them to have been regular upon the 

 body and wing, and crowded into fascicles upon the anterior side. 



Ligamental ai-ea narrow and finely striated. Interior unknown. 



The largest specimen of the left valve observed has a length of 48 mm., 

 height 25 mm., and hinge-line 22 mm. A smaller example has a length of 

 33 mm., height 22 mm., and hinge-line 20 mm. A large right valve has a 

 length of 40 mm., height 25 mm., and hinge-line 27 nun. 



This species differs from L. Mentor in its proportionally longer, narrower and 

 sul)-angular l)ody, the smalliT, less defined wing, the less distinct byssal sinus, and 

 the smaller anterior i'w\. The wing is often imperfect, giving the shell nuicti 

 the aspect of Mytilus or Mnmoi.A. Tlie axis of the shell in the left valve is 

 apparently slightly curved, giving it a characteristic expression. 



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

 nil tlie road from Olean, N. Y., to Smethport, Pa. 



