LAMELLIBRANCHIATA. 169 



Hinge-line straight, less than the length of the shell. 



Beak anterior, acute, rounded, inclined forward, prominent. Uniljonal 

 region gibbous, subtending an acute angle. 



Ear short, directed downward, limited by an undefined sulcus, with 

 scarcely any evidence of a byssal sinus. Wing comparatively small, flat, 

 not strongly defined, joining the body of the shell above the middle of its 

 height ; margin concave ; extremity abruptly acute. 



Test marked by fine concentric strire, which, at intervals, are crowded into 

 fascicles upon the body of the shell, giving an undulating surface. On the 

 wings they are closely and evenly arranged. 



Interior unknown. 



One specimen has a length of 20 mm., height 25 mm., and hinge-line about 

 17 mm. A small specimen has a length of 13 mm., height 15 mm., and 

 hinge-line 12 mm. 



This species is more prominent on the umljo and more convex below than 

 any of the forms described in this group ; the wing is shorter and smaller, 

 with less extension along the postei'ior slope. A young shell referred to this 

 species and in the same association has a body less abruptly defined on the 

 posterior side. 



Formation and locality. In the coarser grits of the Hamilton group, in tlie 

 southern part of Schoharie county, N. Y. 



Leiopteria Gabbi, n. sp. 



PLATE LXXXVm, FIG. 14. 



Shell small ; body ovate, very slightly oblique ; height considerably greater 

 than the length ; anterior margin nearly erect for half the height of the 

 valve, and regularly curving to the post-basal margin, which is scarcely 

 produced. 



Valves sub-equally convex ; the greatest convexity Ijeing a little above the 

 middle in the left valve, and on the umbo of the right valve. 

 Hinge-line not extended, less than the length of the shell. 

 22 



