372 Thirty-fifth Report on" the State Museum. 



where it becomes gibbous. Right valve unknown. Hinge-line 

 straight, a little less than the height of the valve. Beak sub- 

 anterior, directed forward, moderately prominent, scarcely rising 

 above the cardinal line. Umbonal region abruptly gibbous, sub- 

 tending an angle of about 30°. Anterior end short in the direction 

 of the hinge-line, separated from the body by a well-marked byssal 

 depression extending for half the length of the valve ; extremity 

 abruptly angular. Wing large, joining the body below the middle 

 of its length, obscurely defined from the body of the valve; margin 

 moderately oblique, nearly straight, or slightly convex below, with a 

 very gentle concavity just beneath the cardinal line. 



Test thin ; casts of the interior obscurely marked by fine striae of 

 growth which are sometimes fasciculate upon the body of the shell. 



An individual of medium size has a length of 32 mm., height 27 

 mm., and hinge-line 23 mm. A smaller example has a length of 25 

 mm., height 20 mm., and hinge-line 18 mm. 



This species is distinguished from L. Maclurii by its shorter form, 

 less parallel sides of the body, larger wing, which is not extended on 

 the cardinal margin, and the body is not angular along its entire 

 length. 



Formation and locality. In a coarse sandstone of the Upper Che- 

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



Section c, rostrata. 

 Leptodesma extentjatum. 



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

 tions: PI. 22, fig'. 23. Jan., 1883. 



Shell of medium size, elongate, semi-ovate ; body narrowly ovate and 

 oblique to the hinge-line at an angle of about 30°; length nearly 

 twice the height ; ante-by ssal margin very oblique, scarcely depressed 

 at the sinus, thence nearly straight to the broadly curved base ; 

 posterior margin abruptly recurved. Left valve moderately convex 

 below, gibbous on the umbo. Right valve flat below, depressed- 

 convex in the middle, and more convex above, proportionally wider 

 than the left, and distinguished by its depressed-convex form. 

 Hinge-line straight ; length greater than the length of the valve. 

 Beak of left valve sub-anterior, acute, directed forward, rising 

 slightly above the hinge-line. Beak of right valve depressed, not 

 rising above the cardinal line. Umbonal region of left valve 

 moderately gibbous, subtending a very acute angle. Anterior end 



