390 Thibty-fifth Report on the State Museum. 



Leptodesma Phaon. 



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 Unpublished. 



Shell above the medium size, narrow-rhomboidal ; body sub-cylindri- 

 cal, very oblique, making an angle with the hinge-line of less than 

 30°; length nearly twice the height; ante-byssal margin short, 

 oblique, curving into the broad, almost straight ventral side ; poste- 

 rior margin abruptly but regularly rounded and continuous with the 

 base of the wing. Left valve very convex, gibbous above . Right 

 valve unknown. Hinge-line straight, somewhat longer than the 

 height of the shell. Beak at the anterior third of the hinge-line, 

 obtuse, moderately elevated, directed forward. Umbonal region 

 gibbous, not well-defined anteriorly, abruptly depressed on the 

 posterior. Umbonal angle acute. Anterior end short, acute at 

 the extremity, defined by a slight bend in the striae indicating the 

 byssal sinus. Wing small, narrow-triangular, joining the body near 

 the posterior end ; margin straight, very oblique ; extremity angular. 



Test marked by strong concentric striae, which at equal intervals 

 are raised into sharp lamellae, and are more crowded in front. 



A left valve has a length of 47 mm., height 25 mm., and hinge- 

 line 27 mm. 



This species is remarkable for its extreme obliquity of body. In 

 its general features it somewhat resembles L. arciforme, but differs in 

 the less angular gibbosity of the body, larger wing, more extended 

 anterior end, and in the surface markings. 



Formation and locality. In the upper part of the Chemung group 

 at Mansfield, Tioga county, Pa. 



Leptodesma pkopistquum. 



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 Unpublished. 



Shell of medium size, rhomboidal ; body narrow, sub-cylindrical, 

 very oblique, making an angle of about 30° with the hinge-line; 

 length more than twice the height ; anterior margin oblique ; ven- 

 tral margin nearly straight on the anterior half, and broadly curving 

 over the lower half ; posterior margin abruptly curved. Valves 

 gibbous, nearly equal in size and convexity. Hinge-line straight, 

 longer than the height of the valve. Beaks at about the anterior 



