LAMELLIBRANCHIATA. 205 



anterior third of the hinge. Ombonal region narrow and gibbous, subtend- 

 ing an acute angle. 



Anterior end limited by a broad, nearly vertical byssal depression which 

 extends one-half the height of the valve: margin convex; extremity 

 acuminate. Wing triangular, not distinctly limited, joining the body near 

 the posterior extremity; margin oblique, nearly straight, concave just 

 beneath the hinge-line ; extremity mueronate. 



Test thin, marked by fine concentric striaj, which are regular over the 

 body and wing and strongly lamellose and fasciculate on the anterior end. 



Interior unknown. 



A small left valve has a greatest length of 28 mm., height 17 mm., hinge- 

 line 21 mm. 



This species is distinguished from L. Creon and L. Demus by its narrower 

 anterior end and more rounded ventral margin. 



Formation and locality. In the Chemung group, Lawrenceville, Tioga county. 

 Pa. 



Leptodesma Mentor, n. sp. 



ri.ui: \( . FIG :>; and PLATE XXIII, FIU iv- 



Shell of medium size, sub-rhomboidal ; body elongate-ovate, narrow at the 

 anterior end, oblique, making an angle of about 40 3 with the hinge-line ; 

 length one-third greater than the height : ante-byssal margin oblique, curving 

 into the gently depressed sinus : basal margin broadly curving into the 

 rounded posterior extremity. 



Left valve moderately convex in the lower part, becoming convex in the 

 middle and gibbous above. Right valve unknown. 



Hinge-line straight : length about equal to the height of the valve. 



Beak sub-anterior, directed forward, acute, but little elevated above the 

 hinge-line. Umbonal region abruptly, or sub-angularly gibbous, narrow, 

 subtending an acute angle. 



Anterior end small, narrow, limited by an oblique byssal depression which 

 reaches about half way down the body of the shell; extremity pointed. 



