394 Thiety -fifth Report ok the State Museum 



Leptodesma mytiliforme. 



Leptodesma mytiliforme, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 25, figs. 7. 11. Jan., 1883. 



Shell of medium size, elongate-ovate ; body narrowly elliptical, be- 

 coming attenuate in front, oblique, .making an angle of nearly 45° 

 with the hinge-line ; length more than one- third greater than the 

 height; ante-byssal margin oblique, gently curving into a scarcely 

 perceptible sinus, giving a straight or slightly concave margin ex- 

 tending for two-thirds the length of the valve ; posterior margin 

 abruptly rounded. Left valve regularly convex in the lower half, 

 becoming contracted, gibbous 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 um- 

 bonal region narrow and more angular. Hinge-line straight, about 

 equal to the height of the valve. Beaks sub-anterior, directed for- 

 ward, 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 depres- 

 sion 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 strias which have left their im- 

 pression 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. 



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, nar- 

 rower 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 Che- 

 mung group, on the road from Olean, N. Y., to Smethport, Pa. 



