198 
PALEONTOLOGY OF NEW YORK. 
Test thin, marked by concentric striae, which, on the body of the shell, 
are crowded into fascicles and assume a distinct regularity in passing over 
the wing. The hinge shows one or two slender parallel grooves. 
Three similar specimens of the left have respectively the following 
dimensions: length 32, 32 and 33 mm., height 22, 25 and 23 mm., and 
hinge-line 25, 27 and 25 mm. 
In this species the body of the shell resembles L. robustum and L. potens, but 
is less oblique to the hinge-line, the wing less defined, and its posterior 
extremity not produced into a spine. 
Formation and locality. In the upper part of the Chemung group, Lawrence- 
ville, Tioga county, Pa. 
Leptodesma umbonatum. 
PLATE XXII, FIG. 13; and PLATE XC, FIG. 9. 
Leptoclesma umbonatum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PL 22, fig. 13. 
Jan., 1883. 
Shell of medium size, rhomboidal; body oblique, narrowly ovate; greatest length 
one-half more than the height; ante-byssal margin straight or slightly 
convex, curving into the wide sinus, oblique below; ventral margin broadly 
rounded; posterior extremity moderately produced, abruptly recurved. 
Left valve convex, ventricose above the lower third. Right valve 
unknown. 
Hinge-line straight; length less than the length of the shell. 
Beak at about the anterior third of the cardinal line, directed slightly 
forward, prominent and arching over the hinge. Umbonal region narrow 
and ventricose, defined anteriorly by a broad, shallow depression, which is 
nearly vertical to the hinge-line, and posteriorly by the abrupt depression of 
the body. 
Anterior end acute, triangular. Wing rather large, triangular, convex, 
joining the body of the shell near the posterior extremity; margin broadly 
concave; extremity produced, acute. 
