LAMELLIBRANCHIATA. 223 



intervals, have been elevated into fascicles, and on the external shell, were 

 probably sharp laraellose strias. 



Interior unknown. 



The specimen described has a length of 50 nun., height 31 mm., ami 



hinge-line 3-3 mm. 



This species resembles L. Orodes, and may be distinguished by the more 

 extended and acute anterior extremity, and the larger wing which is distinct I \ 

 concave on the margin and recurved just below the cardinal line. It differs 

 from L. Maclurii in its more produced anterior extremity, and broader wing 

 which is less abruptly recurved at the extremity. 



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

 south of Smethport, McKean county, Pa. 



Leptodesma Lesleyi, n. sp. 



PLATE XCI, FIG 7 



Shell large, sub-rhomboidal ; body narrow, elongate-ovate, making an angle of 

 30° with the cardinal line ; length more than one-third greater than the 

 height : ante-byssal margin oblique, nearly straight above, curving into the 

 broad sinus; ventral margin broadly curved ; posterior extremity abruptly 

 rounded. 



Left valve convex below, narrowly gibbous in the middle and on the 

 umbo. Right valve less convex, depressed below. 



Hinge-line straight, nearly equal to the greatest length of the shell. 



Beaks sub-anterior, directed forward,. acute and arching over the hinge in 

 the left valve, depressed in the right valve. Umbonal region of the left 

 valve narrowly gibbous, and subtending a very acute angle. 



Anterior end large, limited by a broad, distinct, oblique byssal depression; 

 extremity flattened and acuminate. Wing large, triangular, joining the 

 body near the post-basal extremity, limited by the abrupt forward bending of 

 the strife ; margin deeply concave : extremity produced and acute. The 



