LAMELLIBRANCHIA TA. 
181 
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 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 robustum. 
PLATE XXI, FIGS. 15, 16, 20; and PLATE LXXXIX, FIG. 8. 
Leptodesma robustum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 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; 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. Right 
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 margin 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 
