LAMELLIB RANCHIA TA. 
195 
which is more deeply concave on the margin and with the extremity more 
produced; the anterior extremity is also larger and apparently not produced. 
The right valves of the two species are very unlike. 
Formation and localities. In the Upper Chemung group at Bradford, Pa., and - 
doubtfully occurring near Elmira, N. Y. 
Leptodesma lepidum. 
PLATE XXI, FIG. 40 ; and PLATE LXXXIX, FIG. 16. 
Leptodesina lepidum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 21, fig. 40. Jan., 1883. 
Shell of medium size, narrow, sub-rhomboidal; body narrowly ovate, somewhat 
straight on the posterior slope, oblique at an angle of about 35° with the 
hinge-line; length less than twice the height; margin very oblique anteriorly, 
curving into a marked byssal sinus; base broadly rounded to the posterior 
extremity, which is abruptly recurved. 
Left valve regularly convex below, gibbous above. Right valve very 
depressed-convex, nearly flat below, moderately convex on the umbo, 
considerably smaller than the left. 
Hinge-line straight; length more than two-thirds the greatest length of 
the shell. 
Beaks sub-anterior, directed forward, prominent in the left valve, depressed 
in the right valve. Umbonal region narrow and abruptly gibbous in the 
left valve; depressed and scarcely defined in the right. Umbonal angle 
about 30°. 
Anterior end small, acute at the extremity, limited by a distinct vertical 
byssal depression. Wing narrow-triangular, joining the body at about one- 
fourth the length from the posterior extremity; margin deeply concave, 
abruptly recurved along the cardinal line, and extended into a spiniform 
process. In the right valve the wing is scarcely defined. 
Test marked by very fine concentric striae, which are regular upon the 
body and wing, and crowded and fasciculate on the anterior. In the right 
valve the striae are more distant and more even in their character. 
