Lamellibranchiata. 381 



but little elevated above the hinge-line in the left valve. Umbonal 

 region narrow, and abruptly gibbous. In the right valve the beak 

 is depressed, and the umbonal region a little less gibbous than in 

 the other valve. Anterior end small, acute at the extremity, rounded 

 below to the distinct byssal sinus. Wing large, wide-triangular, 

 joining the body at the posterior extremity; margin moderately 

 concave, the greatest concavity being above the middle, from which 

 point it turns abruptly outward and the extremity is produced into 

 a spiniform extension. In the right valve the wing is less distinctly 

 limited from the body than in the left. 



Test thin, marked by sharp, elevated, concentric striae, with finer 

 intermediate lines of growth. The stronger striae are somewhat 

 regular upon the body and wing, while on the anterior side they are 

 crowded and fasciculate. Muscular impression large, sub-circular, 

 situated below the middle of the posterior slope. The pallial line 

 extends from the lower part of the muscular impression, parallel to 

 the basal and anterior margins, into the cavity of the beak. Liga- 

 mental 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, Bradford county, Pa. 



Leptodesma Onus. 



Leptodesma Orus, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell above the medium size, sub-rhomboidal ; body broadly ova'te, 

 abruptly contracted toward 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 ex- 

 tremity, 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 ex- 

 tremity, defined by a marked byssal depression. Wing large, 

 triangular, without distinct limitation, joining the body near the 

 posterior extremity ; margin oblique below, concave above the mid- 

 dle, turning abruptly outward and terminating in a sharply angular 

 extension of the cardinal line. 



