LAMELLIBRANCHIA TA. 
203 
narrow, limited by the abrupt slope of the posterior side. Umbonal angle 
acute. 
Anterior end large, short, rounded, curving into the byssal sinus, defined 
by a marked byssal depression; extremity acute. Byssal sinus marked by a 
broad, shallow curvature of the margin. Wing small, narrow-triangular, not 
distinctly limited; margin concave; extremity acute. The wing of the 
right valve is comparatively larger. 
Test thin, marked by fine concentric lines of growth, which are some¬ 
times regular or often crowded into fascicles on the body of the shell. 
Cardinal line marked by a narrow longitudinal groove. Interior unknown. 
A left valve has a length of 32 mm., height 21 mm., hinge-line 23 mm. 
A smaller specimen has a length of 29 mm., height 20 mm., hinge-line 
22 mm. 
This species resembles in general expression L. umbonatum var. depressum, 
but it is uniformly smaller, the anterior end shorter, byssal depression nar¬ 
rower, wing smaller, the recurving of the posterior margin of the body less 
abrupt, the limits of the wing less distinctly marked, the extremity less 
produced, and the umbonal region not so abruptly gibbous. 
Formation and locality. In the upper part of the Chemung group, Lawrence- 
ville, Tioga county, Pa. 
Leptodesma Demus, n. sp. 
PLATE XC, FIGS. 15, 16. 
Shell of medium size, rhomboidal; body very oblique, narrowly ovate; 
extreme length more than one-third greater than the height; ante-byssal 
margin oblique, rounded below, concave at the sinus; basal margin broadly 
rounded; posterior margin abruptly curved. 
Left valve convex, gibbous above. Right valve of similar form, less 
convex, and in the posterior part wider and more depressed. 
Hinge-line straight, about two-thirds as long as the length of the valve. 
Beaks situated at about the anterior third of the hinge, prominent, 
