236 
PALAEONTOLOGY OF NEW YORK. 
Left valve regularly convex in the lower half, becoming contracted, gib¬ 
bous and sub-angular above the middle. The right valve is smaller, the 
hinge-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 forward, acute, scarcely raised above .the 
hinge-line. Umbonal region narrow and gibbous, subtending a very acute 
angle. 
Anterior end short, abruptly attenuate, acute, limited by a shallow byssal 
depression which extends for half 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 striae 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 area 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 mm. 
This species differs from L. Mentor in its proportionally longer, narrower and 
sub-angular body, the smaller, less defined wing, the less distinct byssal sinus, and 
the smaller anterior end. The wing is often imperfect, giving the shell much 
the aspect of Mytilus or Modiola. The 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, 
on the road from Olean, N. Y., to Smethport, Pa. 
