LAMELLIBRANCHIATA. 197 



Test moderately thick, marked by elevated lamellose stria-, with liner 

 intermediate lines. 



Interior unknown. 



The specimen described has a length of 4"J mm., height '24 mm., hinge- 

 line, to base of spine. 30 nun. The spine is probably extended from 5 to 

 1 1) mm. beyond. 



This species resembles L. lepidum, hut differs in its more gibbous form, larger 

 and more obtuse anterior extremity. 



Formation and locality. In sandstones of the Upper Chemung group, Mckean 

 county, Pa., on the road from Bradford to Farmers' Valley, Cattaraugus 

 county, N. Y. 



Section b, umbonata. 

 Leptodesma Medon, n. sp. 



PLATE X( , FIGS. 1-1. 



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

 of about (id with the hinge-line; length nearly one-third greater than the 

 height; ante-byssal margin curving slightly outward, concave at the sinus; 

 basal and posterior margins broadly rounded, passing directly into the wing. 



Left valve gibbous above, depressed-convex below. Right valve some- 

 what 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. 



