362 Thirty-fifth Keport on the State Museum. 



Test thin, marked by fine concentric striae which are fasciculate 

 on the body of the shell, producing an undulated appearance. The 

 striae are sub-lamellose on the anterior and regular over the wing. 



A left valve has a length of 35 mm., height 23 mm., and hinge- 

 line about 28 mm. A similar right valve has a length of 29 mm., 

 and height 20 mm. 



This species differs from L. robustum in its comparatively larger 

 wing, without distinct sinus in the margin, and the more produced 

 anterior end. It resembles L. potens, but differs in its more produced 

 anterior end, less gibbous body, and more abruptly recurved posterior 

 margin. 



Formation and locality. In the upper beds of the Chemung group, 

 near Bradford, McKean county, Pa. 



Leptodesma Stephani. 



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 Unpublished. 



Shell above the medium size, sub-rhomboidal ; body ovate, straight 

 on the post-umbonal slope, oblique at an angle of from 30° to 40° 

 with the hinge-line ; height about five-eighths of the length; ante- 

 byssal margin vertically truncate above, curving below into the 

 sinus ; basal margin broadly rounded, abruptly recurved at the post- 

 basal extremity. Left valve convex, somewhat gibbous above. 

 Eight valve depressed-convex below and abruptly gibbous at the 

 umbo. Hinge-line straight, about five-sixths the length of the 

 valve. Beaks sub-anterior, directed slightly forward, obtuse, 

 prominent, rising a little above the cardinal line. Umbonal region 

 regularly gibbous, subtending an acute angle. Anterior end large, 

 short, defined by a nearly vertical byssal depression, which reaches 

 the margin about one-third the length of the valve from the ante- 

 rior extremity, which is abruptly rounded or truncate. Wing join- 

 ing the body at less than one-fourth the length of the valve from 

 the posterior extremity ; margin very oblique below, deeply concave 

 above ; the greatest depth of the concavity is above the middle, 

 whence the margin is abruptly recurved and produced into a spini- 

 form extension. 



Test of moderate thickness, marked by nearly equi-distant lamel- 

 lose concentric striae, with intermediate finer striae of growth. The 

 lamellose striae are subdued upon the wing, and very strong and 

 irregular on the anterior side. 



