LAMELLIBRANCHIATA. 207 



Anterior end short in the direction of the hinge-line, separated from the 

 body by a well-marked byssal depression extending for half the height 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 strise of growth 

 which are sometimes fasciculate upon the body of the shell. 



Interior unknown. 



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. Madurii 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. 



The specimen, PI. xxv, fig. 10, has been flattened by pressure and is referred 

 with doubt to this species. It has the same general aspect, but is proportionally 

 wider below the middle, and more broadly rounded on the post-basal margin, 

 which peculiarities may have been produced by compression. 



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

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



Section c, rostrata. 

 Leptodesma extenuatum. 



PLATE XXII, FIG. 23 ; ANlJ PLATE XC, FIGS. 17, 18. 



Leptodesma extenuatum, Halt,. Pal. N. Y., x-ol. v, pt. 1. Plates and Exiilanatioiis : 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-byssal margin very oblique, scarcely depressed at the sinus, thence nearly 

 straight to the broadly curved jjase ; posterior margin abruptly recurved. 



