228 
PALEONTOLOGY OF NEW YORK. 
region indistinctly defined anteriorly; the posterior side limited by the post- 
nmbonal depression. 
Anterior end short; extremity angular. Wing undefined, joining the 
body one-third of the length from the posterior extremity; margin gently 
concave; extremity scarcely produced. 
Test marked by fine, sharply elevated, concentric striae, which are more 
crowded anteriorly, and very regular on the wing. 
Interior unknown. 
The specimen described, which is a separate left valve, has a length of 
28 mm., height 20 mm., and hinge-line 18 mm. 
Formation and locality. In the shales of the Chemung group at Philips- 
burg, N. Y. 
Section e, arcoidea. 
Leptodesma Maclurii. 
PLATE XXV, FIGS. 8, 13 ; and PLATE XCI, FIGS. 13, 14. 
Leptodesma Maclurii, Hall. Pal. N. Y., vol'. v, pt. 1. Plates and Explanations: PI. 25, figs. 8, 18. 
Jan., 1883. 
Shell large, rhomboidal; body elongate, sub-elliptical, oblique, making an angle 
of about 45° with the hinge-line; length more than one-third greater than 
the height; ante-byssal margin oblique, gently curving into a long, shallow, 
undefined sinus; ventral margin nearly straight, curving abruptly into the 
posterior margin, which is rounded and abruptly recurved. 
Left valve moderately convex in the lower part, becoming gibbous and 
sub-angular along the line of the axis to the beak. Right valve distinctly 
arcuate, less convex, narrowly gibbous in the umbonal region. 
Hinge-line straight; length about equal to the height of the valve. 
Beaks sub-anterior, directed forward, but little elevated above the hinge¬ 
line in the left waive, depressed in the right. Umbonal angle acute. 
Anterior end narrow-triangular, limited by an oblique depression which 
extends nearly one-third of the length of the valve to the byssal sinus. 
