Lamellibranchiata. 369- 



Left valye convex below, gibbous above. Right valve less gibbous 

 than the left and more expanded. Hinge-line straight ; length a 

 little greater than the height of the shell. Beaks at about the 

 anterior third of the hinge-line, sub-acute, directed forward, 

 prominent, arching over the hinge. Umbonal region very gibbous, 

 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 

 sometimes regular or often crowded into fascicles on the body of 

 the shell. Cardinal line marked by a narrow longitudinal groove. 



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 narrower, wing smaller, the recurving of> the posterior 

 margin of the body less abrupt, the limits of the wing less distinctly 

 marked, the extremity Jess produced, and the umbonal region not so 

 abruptly gibbous. 



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

 Lawrenceville, Tioga county, Pa. 



Leptodesma Demus. 



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 Unpublished. 



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, 

 directed forward and rising a little above the cardinal line. Um- 

 bonal region gibbous, defined on the anterior by the byssal depres- 

 sion, and on the posterior by the slope of the side. Anterior end 

 large, limited by an oblique byssal depression, angular at the 

 extremity and rounded below. Byssal sinus extending to a point 

 fSen. Doc. No. 38.] 47 



