LAMELLIBRANCHIA TA. 
219 
Muscular impression large, sub-circular, situated below the middle of the 
posterior slope. The pallial line extends from the lower part of the muscu¬ 
lar impression, parallel to the basal and anterior margins, into the cavity of 
the beak. Ligamental area wide. 
A left valve of this species has a length of 35 mm., height 26 mm., and 
hinge-line 33 mm. A somewhat larger right valve has a length of 38 mm., 
height 28 mm., and hinge-line 33 mm. 
Formation and locality. In the shales of the Chemung group at Canton, Brad¬ 
ford county, Pa. 
Leptodesma Orus, n. sp. 
PLATE XC, FIG. 30. 
Shell above the medium size, sub-rhomboidal; body broadly ovate, abruptly 
contracted towards the anterior end, oblique at an angle of about 50° with 
the hinge-line; ante-byssal margin oblique, curving into the marked sinus, 
thence gently curving to the posterior extremity, which is broadly rounded. 
Left valve depressed-convex below, moderately convex above. Right 
valve unknown. 
Hinge-line straight; length about five-sixths the greatest length of the 
valve. 
• Beak sub-anterior, directed forward, slightly rising above the cardinal line. 
Umbonal region moderately gibbous, subtending an acute angle. 
Anterior end triangular, abruptly acute at the extremity, defined by a 
marked byssal depression. Wing large, triangular, without distinct limita¬ 
tion, joining the body near the posterior extremity; margin oblique below, 
concave above the middle, turning abruptly outward and terminating in a 
sharply angular extension of the cardinal line. 
Test thin, marked by sharp, elevated, concentric striae with finer inter¬ 
mediate ones. The stronger striae are regular on the wing and posterior 
side; irregular and fasciculate on the anterior. 
Interior unknown. 
