180 
PALEONTOLOGY OF NEW YORK. 
valves are the same in form; the sinus in the wing of the right valve is not 
so deep as in the left valve, but much deeper than in the right valve of that 
species. 
The original specimen is illustrated in figure 14 of plate xxi, and figures 17, 
18, 19 are right valves occurring in the same beds. 
This form is associated with Avicula spinigera of Conrad, which is a smaller 
shell, less oblique, narrower, and with a more angular anterior extremity. 
Formation and localities. In the Chemung group, at Painted Post, Steuben 
county, between Corning and Elmira, and at Chemung Narrows, N. Y. 
Leptodesma Shumardi, n. sp, 
PLATE LXXXIX, FIGS. 5, 6. 
Shell of medium size, sub-rhomboidal; body narrow, very obliquely-ovate; 
length somewhat less than twice the height; margin very broadly curving 
from the anterior extremity to the base ; basal 'margin rounded and abruptly 
recurved at the post-basal extremity. 
Left valve quite convex; the greatest convexity about one-third the length 
of the valve below the apex. Right valve unknown. 
Hinge-line straight; length about equal to the height of the shell. 
Beak sub-anterior, obtuse, prominent and arching over the hinge-line. 
Umbonal region gibbous, subtending an acute angle. 
Anterior extremity short, acute, straight above, oblique below. Wing 
small, narrow-triangular, extending to a point below the middle of the length 
of the shell; margin concave; extremity acute, probably spiniform. 
Test thin, marked by lamellose, often fasciculate striae, which are more 
crowded on the anterior side and byssal depression, and more regular and 
distinct on the wing. 
Interior unknown. 
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. 
