LAMELLIBRAN CHI A TA. 
221 
The specimen described is 60 mm. in length, with a height of 29 mm., 
and hinge-line 61 mm. 
This species bears considerable resemblance to L. extenuatum in the characters 
of the right valve; but it is proportionally narrower, the posterior extremity 
more abruptly recurved and the hinge-line less extended behind. A single 
right valve is the only specimen at present known. It differs sufficiently from 
the other forms here described to be easily recognized. 
Formation and locality. In a white sandstone of the middle portion of the 
Chemung group near Angelica, Alleghany county, N. Y. 
Leptodesma rude. 
PLATE XXV, FIG. P2; and PLATE XCI, FIG. 3. 
Leptodesma rude. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 25, fig. 12. Jan., 1883. 
Shell large, sub-rhomboidal; body broadly ovate below, rapidly attenuating 
above, oblique to the hinge-line at an angle of about 50°; length less than 
one-third greater than the height; ante-byssal margin curving into the broad 
sinus; ventral and posterior margins broadly rounded. 
Left valve convex below, becoming narrow and gibbous above the middle. 
Right valve less convex. 
Hinge-line straight, less than the length of the valve. 
Beak sub-anterior, directed forward, acute, little elevated above the 
cardinal line. Umbonal region narrow and gibbous, subtending an acute 
angle. 
Anterior end large, limited by a vertical byssal depression; extremity 
acute. Wing of moderate size, joining the body at the posterior extremity; 
margin concave; termination acute. 
Test thick, marked by concentric striae, which have been fasciculate, 
leaving strong undulations upon the cast of the interior. These surface 
characters are subdued in the right valve. 
Interior unknown. 
