LAMELLIBRANCHIATA. 203 



narrow, limited by the abrupt slope of the posterior side. Umbonal angle 

 acute. 



Anterior end large, short, rouuded, curving into the bjssal 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 



'>'? 



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 n<it 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, riGS, I.-,, n;. 



Shell of medium size, rhomboidal ; body very oblique, narrowly ovate ; 

 extreme length more than one-third greater than the height ; ante-byssal 

 margin ol)li([ue, 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, 



