()38 Systematic Paleoxtoi.ogy — Upper Devonian 



iiiiU'oin l)nia(lly romid^'d below the bvssal sinus; ventral niaririn I'egularly 

 curved. Anterior M-ing short, hardly depressed, obtuse. Posterior wing 

 not extended beyond the body of the shell. Surface Avith concentric 

 growth lines. 



Length 25 mm.; height 2"3 mm. 



Occurrence. — Jennings Formation, Chemung Member. Eoad north 

 of Deer Park Station; Allegany Grove, 2355 ?. 



Collection. — Maryland Geological Survey. 



Leptodesma medon Hall 



Plate LXIII, Figs. 11-15 



Leptodesma medon Hall, 1884, Pal. of N. Y., vol. v, pt. 1, sec. i, p. 197, pi. xc, 

 figs, 1-4. 



Description. — "Shell of medium size, subrhomboidal; body broad- 

 ovate, oblique at an angle of about 60° with the hinge-line; length nearly 

 one-third greater than the height ; antebyssal margin curving slightly 

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

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

 pressed-convex below. Eight valve somewhat less convex than the left. 

 The riglit \alve 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; ex- 

 tremity acute. Test thin, marked by concentric strige, which, on the 

 bodv of the shell, are crowded into fascicles and assume a distinct regu- 

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

 parallel grooves. Three similar specimens of the left 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. rohustiim and L. potens, but is less oblique to the 



