LAMELL1BRANCHIATA. 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 nun., 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. 



PL \tk \< . hi, 30. 



Shell above the medium size, sub-rhomboidal ; body broadly ovate, abruptly 

 contracted towards the anterior end, oblique at an angle of about 50 c witb 

 the hinge-line; ante-ljyssal 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 live-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. 



