Lamellibranchiata. 365 



Section h, umbonata. 



Leptodesma Medon. 



Lepiodesma Medon, Hall. Pal. N. Y.,vol. v, pt. 1. Unpublished. 



Shell of medium size, sub-rhomboidal ; body broad-ovate, oblique at 

 an angle of about 60° with the hinge-line; length nearly one- third 

 greater than the height ; ante-byssal margin curving slightly out- 

 ward, concave at the sinus; basal and posterior margins broadly 

 rounded, passing directly into the wing. Left valve gibbous above, 

 depressed-convex below. Eight valve somewhat less convex than 

 the left. The right valve appears to have been somewhat smaller 

 and the base more extended than in the left valve. Hinge-line 

 straight ; length a little greater than the height of the shell. Beaks 

 at about the anterior third of the hinge, acute, prominent, arching 

 over the hinge-line. Umbonal region gibbous, descending almost 

 vertically on the posterior, and sloping abruptly on the anterior 

 side. Umbonal angle acute. Anterior end short, separated from 

 the body by a marked sinus; extremity angular, gently rounded 

 below. Wing not defined, broad-triangular, reaching nearly to the 

 posterior end of the body ; margin slightly concave ; extremity acute. 



Test thin, marked by concentric striae, which, on the body of the 

 shell, are crowded into fascicles and assume a distinct regularity in 

 passing over the wing. The hinge shows one or two slender parallel 

 grooves. 



Three similar specimens of the left valve have respectively the 

 following dimensions : length 32, 32 and 33 mm,, height 22, 25 and 23 

 mm., and hinge-line 25, 27 and 25 mm. 



In this species the body of the shell resembles L. robustum and L. 

 potens, but is less oblique to the hinge-line, the wing less defined, and 

 its posterior extremity not produced into a spine. 



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

 Lawrenceville, Tioga county, Pa. 



Leptodesma umbonatum. 



Leptodesma umbonatum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 22, fig. 13. Jan., 1883. 



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

 greatest length one-half more than the height ; ante-byssal margin 

 straight or slightly convex, curving into the wide sinus, oblique 

 below ; ventral margin broadly rounded ; posterior extremity 

 moderately produced, abruptly recurved. Left valve convex, ven- 



