LAMELLIBRANCHIATA. 
205 
anterior third of the hinge. Umbonal 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 mucronate. 
Test thin, marked by fine concentric striae, 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. 
PLATE XC, FIG. 5; and PLATE XXIII, FIG. 15? 
Shell of medium size, sub-rhomboidal; body elongate-ovate, narrow at the 
anterior end, oblique, making an angle of about 40° 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. 
