Lamellibranchiata. 363 



A large specimen of this species has a length of 43 mm., height 

 25 mm., and hinge-line about 33 mm. A somewhat smaller example 

 has a length of 37 mm., height 25 mm., and hinge-line 30 mm. A 

 right valve measures 33 mm. in length, 24 mm. in height, and 

 hinge-line 28 mm. 



This species resembles L. Matheri, with which it is associated, but 

 differs in its greater obliquity, more elongate outline, more extended 

 and flatter wing, 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 Brad- 

 ford, Pa., and doubtfully occurring near Elmira, N. Y. 



Leptodesma lepidum. 



Leptodesma 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. Eight 

 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 ex- 

 tremity, 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 pro- 

 cess. 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. 



A left valve of medium size has a length of 41 mm., height 22 

 mm., length of the cardinal line about 35 mm. 



