384 Thirty-fifth Be port on the State Museum. 



shallow sinus, which impresses the margin for more than half the 

 length of the valve ; basal and posterior margins broadly rounded. 

 Left valve regularly and gently convex in the lower part, becoming 

 more convex and gibbous above the middle. Right valve unknown. 

 Hinge-line straight, greater than the height of the valve. Beak 

 sub-anterior, directed forward, but little elevated above the hinge- 

 line. Umbonal region abruptly gibbous, subtending an acute angle. 

 Anterior end produced, narrow, acute at the extremity, limited by 

 an oblique, shallow byssal depression. Wing large, joining the body 

 at the posterior end, not distinctly separated; margin slightly 

 oblique below, genfly concave above the middle, and curving slightly 

 outward just below the cardinal line. 



Test thin ; cast marked by fine concentric striae of growth, which, 

 at intervals, have been elevated into fascicles, which, on the external 

 shell, were probably sharp lamellose striae. 



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

 and hinge-line 35 mm. . 



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

 more extended and acute anterior extremity, and the larger wing 

 which is distinctly concave on the margin and recurved just below the 

 cardinal line. It differs from L. Maclurii in its more produced ante- 

 rior extremity, and broader wing which is less abruptly recurved at 

 the extremity. 



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

 mung group, south of Smethport, McKean county, Pa. 



Leptodesma Lesleyi. 

 Leptodesma Lesleyi, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



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- 



