LAMELLIBRANCHIA TA . 
193 
Leptodesma Mathebi. 
PLATE XXXI, FIGS. 8, 9, 10; AND PLATE LXXXIX, FIG. 25. 
Leptodesma Matheri, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 22, figs. 8-10. 
Jan., 1883. 
Shell of medium size, rhomboidal; body ovate, oblique at an angle of 45° with 
the hinge-line; length about one-half greater than the height; ante-byssal 
margin vertical in front, joining the broad curvature of the basal margin; 
posterior margin produced and abruptly recurved. 
Left valve moderately convex below, scarcely gibbous above. Right valve 
shorter and less convex. 
Hinge-line straight; length much less than the length of the valve, but 
nearly equal to the height. 
Beaks at about the anterior third of the hinge, obtuse, directed slightly 
forward, not very prominent. Umbonal region moderately gibbous, sub¬ 
tending an acute angle. 
Anterior end produced, flattened, acute at the extremity. Wing of 
moderate dimensions, extending nearly to the posterior extremity; margin 
gently concave; extremity produced and acute. 
Test thin, marked by fine concentric striae which are fasciculate on the 
body of the shell, producing an undulated appearance. The striae are 
sub-lamellose on the anterior and regular over the wing. 
Interior unknown. 
A left valve has a length of 35 mm., height 23 mm., and hinge-line about 
28 mm. A similar right valve has a length of 29 mm., and height 20 mm. 
This species differs from L. robustum in its comparatively larger wing, without 
distinct sinus in the margin, and the more produced anterior end. It resem¬ 
bles L. potens , but differs in its more produced anterior end, less gibbous body, 
and more abruptly recurved posterior margin. 
Formation and locality. In the upper beds of the Chemung group, near 
Bradford, McKean county, Pa. 
25 
