186 
PALAEONTOLOGY OF NEW YORK. 
and the basal margin and the striae along the posterior slope are not so abruptly 
recurved. 
Formation and localities. In the Chemung group, near Corning, Chemung 
county, and Portville, Alleghany county, N. Y. 
Leptodesma disparile. 
PLATE XXV, FIGS. 2-4; AND PLATE LXXXIX, FIGS. 23, 24. 
Leptodesma disparile, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 25, figs. 2-4. 
Jan., 1883. 
Shell of small or medium size, sub-rhomboidal; body ovate, oblique to the hinge 
at about 45°; length more than one-third greater than the height in the 
left valve; ante-byssal margin oblique, gently curving into the sinus; basal 
and posterior margins regularly rounded. 
Left valve convex below, gibbous above. Right valve flat or concave 
below, depressed-convex in the middle, and gibbous on the umbo. 
Hinge-line straight, greater than the length of the shell. 
Beak sub-anterior, prominent, rising above the hinge-line, directed slightly 
forward. Umbonal region gibbous, subtending an acute angle. 
Anterior end large, somewhat produced, limited by a nearly vertical byssal 
depression ; extremity obtusely angular. Wing somewhat large, joining the 
body near the posterior extremity ; margin oblique below, concave above the 
middle, and produced into an elongate spiniform process. The right valve is 
smaller, the wing more extended and less defined, while the anterior end is 
large and well defined by the byssal sinus and sulcus. 
Test marked by concentric, lamellose ridges with intermediate finer lines 
of growth; the lamellse are sometimes more distinct on the right valve. 
Interior unknown. 
A left valve has a length of 28 mm., height 20 mm., hinge-line more than 
30 mm. A right valve has a length of 30 mm., height 18 mm., hinge-line 
33 mm. 
