LA MELLIBRANCHIA TA. 201 



Anterior end short, acute. Wing large, not distinctly limited, extending 

 almost to the posterior extremity; margin scarcely concave below; 

 extremity produced, acute. 



Test marked by fine concentric stria', which are crowded into fascicles at 

 nearly equal intervals, rounded upon the upper part of the body, and sub-angular 

 on the lower part. The striae are crowded and lamellose on the anterior; 

 on the posterior slope they make a short abrupt curve, passing over the u ing 

 with a gently forward direction and curving backward just below the hinge- 

 line. In the weathered surface of the right valve the concentric undulations 

 are stronger, the post-cardinal slope is marked by strong interrupted radii 

 which appear to belong to the intimate structure of the shell. The hinge is 

 marked by a single narrow groove. 



A specimen of the left valve has a length of 22 mm., height 14 mm., and 

 hinge-line about 21 mm. A right valve occurring in the same association 

 has a length, from beak to base, of 26 mm., height 18 nun., and hinge-line 

 about 25 mm. 



This species is quite unlike any here described. 



Formation and locality. In the low T er beds of the Chemung group at the 

 Inclined Plane, near Ithaca, N. Y. 



Leptodesma Caumds, n. sp. 



PLATE XC. FIGS. 6, 7. 



Shell of medium size, rhomboidal ; body oblique, narrowly ovate or sub- 

 cylindrical; length nearly one-third greater than the height; ante-byssal 

 margin somewhat vertically truncate, sloping below into the broadly curving 

 base; posterior margin abruptly curved. 



Valves convex, gibbous above; the right valve smaller than the left. 



Hinge-line straight, nearly equal to the height of the shell. 



Beaks at about the anterior third of the hinge-line acute, directed forward, 

 prominent, arching over the cardinal line. Umbonal region abruptly gibbous, 

 subtending an acute angle. 

 26 



