Lamellibranchiata. 343 



produced. Left valve very convex ; gibbous from below the middle 

 to the umbo. Eight valve unknown. Hinge-line straight, less than the 

 length of the shell. Beak anterior, acute, rounded, inclined forward, 

 prominent. Umbonal 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 striae, which, at intervals, are 

 crowded into fascicles upon the body of the shell, giving an undu- 

 lating surface. On the wings they are closely and evenly arranged. 



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 umbo and more convex be- 

 low than any of the forms described in this group ; the wing is shorter 

 and smaller, with less extension along the posterior 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 the southern part of Schoharie county, N. Y. 



Leiopteria GrABBI. 



Leiopteria Gabbi, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



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 being 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. Beaks acute, prominent, directed forward, 

 anterior to the middle of the shell. Umbonal region gibbous, sub- 

 tending an acute angle. Ear small, straight on the upper margin ; 

 marked by an oblique fold, and separated from the body by an abrupt 

 sulcus and a well-defined byssal sinus. Wing small, triangular, join- 

 ing the body of the shell below the middle of its height, defined by 

 a change in the direction of the surface striae ; margin gently con- 

 cave ; extremity acute. 



Test thin, marked by fine concentric striae, and at regular inter- 

 vals by stronger elevated striae, which give to the macerated shell, 



