LAMELLIBRANCHIATA. 201 



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

 almost ti) the posterior extremity; margin scarcely concave below; 

 extremity produced, acute. 



Test marked by fine concentric striae, Avhich are crowded into fascicles at 

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

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

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

 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 b}' strong interrupted radii 

 which appear to belong to the intimate structm-e 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 l)eak to base, of 2(i mm., height 18 mm., and liinge-line 

 about 2f5 mm. 



This species is quite unlike any here described. 



Formation and locality. In the lower beds of the Chemung group at the 

 Inclined Plane, near Ithaca, N. Y. 



Leptodesma Cadmus, n. sp. 



PLATE XC, KIGS. 11, 7. 



Shell of niediiun size, rhomboidal ; l)ody oljliquo, narrowly ovate or sul)- 

 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 



