LAMELLIBRANCHIA TA. 
215 
Formation and locality. In the Upper Chemung group, Mansfield, Tioga 
county, Pa. 
Leptodesma Orcus, n. sp. 
PLATE XC, PIG. 23. 
Shell of medium size, sub-rhomboidal; body ovate, straight on the posterior 
slope, oblique to the hinge-line at an angle of about 40°; length about one- 
third greater than the height; ante-byssal margin oblique, slightly impressed 
• at the sinus, nearly straight along the base and regularly rounded posteriorly. 
Left valve symmetrically and gently convex below, slightly gibbous 
above. Right valve unknown. 
Hinge-line straight; length equal to three-fourths the length of the shell. 
Beak anterior, acute, little elevated above the hinge-line. Umbonal region 
narrowly gibbous, subtending a very acute angle. 
Anterior end short, limited by an oblique byssal depression; extremity 
acute. Wing of medium size, joining the body about three-fourths its length 
from the beak ; margin abruptly curving forward from the base to near the 
hinge-line, where it is sharply recurved into a short, angular extension of 
the cardinal line. 
Test thin, marked by fine concentric striae, which are somewhat fascicu¬ 
late on the Uody and anterior part of the shell, and a little more sharply 
elevated on the wing. 
Interior unknown. 
The specimen illustrated has a length of 30 mm., height 22 mm., and 
hinge-line 24 mm. 
The form and proportions of body are quite similar to L. Lysander , but it 
is wider on the posterior, narrower on the anterior end, and more oblique to 
the hinge; the striae less sharply elevated, curving more forward along the 
posterior slope, and more abruptly recurved under the cardinal line. 
Formation and locality. In the Chemung group at Kelly’s creek, Tioga county, 
Pa. 
