LAMELLIBRANCHIATA. 195 



which is more deeply concave on the margin and with tlie extremity more 

 produced ; the anterior extremity is also larger and apparently not produced. 

 The right valves of the two sjjecies are very unlike. 



Formation and localities. In the Upper Chemung group at Bradford, Pa., and 

 douhtfully occurring near Elmira, N. Y. 



Leptodesma lepidum. 



PLATE XXI, FIG. 411; AKI) I'LATE LXXXIX, FIG. W. 



LeptodesiiM lepidum. H.\Lr.. Piil. N. Y.. \(.l. \'. ]it. 1. Plates and Explanations: PI. 21. fif?-. 40. Jan., 1883. 



Shell of medium size, narrow, sub-rhomboidal ; body narrowly ovate, somewhat 

 straight on the posterior slope, oldique 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 l^roadly rounded to the posterior 

 extremity, which is abruptly recurved. 



Left valve regularly convex below, gibbous above. Right valve very 

 depressed-convex, nearly flat lielow, 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 

 pi'ocess. 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 strias are more distant and more even in their character. 



