352 Thirty -fifth Report on the State Museum. 



more crowded on the anterior side and byssal depression, and more 

 regular and distinct on the wing. 



Three specimens of left valves have respectively the following 

 dimensions : length 42, 40 and 36 mm., height 24, 27 and 24 mm., 

 and hinge-line about 26, 24 and 24 mm. 



This species differs from L. longispinum in the wider and somewhat 

 less oblique body, with more regularly rounded posterior extremity, 

 the sinus in the wing is less acute, and is farther below the cardinal line; 

 also in the absence of the strong posterior spine of that species. 



Formation and locality. In the Chemung group, near Elmira, N. Y. 



LEPTODESMA ROBUSTU3I. 



Leptodesma robustum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions . PI. 21, figs. 15, 16, 20. Jan., 1883. 



Shell large, sub-rhomboidal ; body obliquely ovate; length much 

 greater than the height ; ante-byssal margin rounded ; margin 

 straight below, or scarcely impressed by the sinus, making a broad 

 curve around the base to the post-basal extremity and thence 

 abruptly recurving toward the beak. Left valve gibbous above, 

 regularly convex below the middle; the greatest convexity about 

 one-third the length of the valve from the beak. Eight valve 

 moderately convex in the middle, flat below and scarcely gibbous on 

 the umbo. Hinge-line straight, about equal to the length of the 

 shell. Beaks nearly anterior, obtuse, sub-erect, very prominent, 

 rising above the hinge-line. Umbonal region gibbous, prominent, 

 abruptly elevated along the post-umbonal side for more than half 

 the length of the shell, then merged in the general convexity ; on 

 the anterior side obscurely limited by a broad undefined depression. 

 Umbonal angle acute. Anterior extremity scarcely auriculate. 

 Wing small, narrow-triangular, extending about half the length of 

 the valve, defined by the curving of the striae ; margin strongly 

 concave, the deepest concavity just below the hinge ; cardinal mar- 

 gin thickened and extended into a strong spine. In the right valve 

 • the wing is larger and broader, and the sinus less deep ; the anterior 

 extremity corresponds with that of the left valve. 



Test moderately thick, marked by fine concentric striae which 

 become more numerous and fasciculate at irregular intervals, pro- 

 ducing a slightly undulated surface, crowded and lamellose on the 

 anterior. 



A specimen of a left valve has a length, from beak to base, of 43 

 mm., height 29 mm., and hinge-line, to the base of the spine, 25 mm. 



