380 Thirty-fifth Keport on the State Museum. 



of the valve. Beaks sub-anterior, acute, directed forward, rising 

 slightly above the hinge-line. Umbonal region gibbous in the left 

 valve, narrower but equally gibbous in the right valve. Umbonal 

 angle acute. Anterior end small, abruptly curving into the byssal 

 depression ; extremity acute, not prolonged. Wing broad, extend- 

 ing along the body about four-fifths of its length from the beak ; 

 margin very oblique in the lower part and moderately concave above 

 the center, then turning abruptly backward just below the cardinal 

 line, and produced into a short spiniform extension. In the right 

 valve the wing is less defined and less concave on the margin. 



Test thin, marked by concentric striae, which are fasciculate on 

 the body and anterior part, and more elevated, distant and sharp 

 upon the wing. Muscular impression large, sub-circular, situated 

 on the posterior slope below the middle of the length. The pallial 

 line extends from the lower part of the muscular impression nearly 

 parallel to the anterior and basal margin. The hinge of the left 

 valve has a strong groove extending its entire length; the right 

 valve has an oblique posterior fold. 



One specimen has a length of 35 mm., height 26 mm., hinge-line 

 31 mm. A similar example has a length of 34 mm., height 24 mm., 

 and hinge-line 28 mm. A right valve has- a greatest length of 31 

 mm., height 20 mm., and hinge-line 25 mm. 



This species resembles L. Orcus, but the wing is wider, less deeply 

 and abruptly sinuate, and the striae turn more abruptly forward at the 

 junction of the body and wing. The body is narrower and more 

 oblique than in L. Lysander, and the anterior end less prolonged. 



Formation and locality. In the shales of the Upper Chemung 

 group near Canton, Bradford county, Pa. 



Leptodesma alatum. 



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 Unpublished. 



Shell of medium size, sub-rhomboidal ; body ovate, rapidly narrowing 

 toward the beak, oblique, making an angle of about 50° with the 

 hinge-line ; length one-third greater than the height ; ante-byssal 

 margin oblique and curving into the distinct sinus ; ventral margin 

 gently curved, joining the broad, rounded, posterior extremity. 

 Left valve gently convex below, becoming more convex and slightly 

 gibbous above the middle. Right valve depressed in the lower por- 

 tion; in the middle and above about equally convex with the left. 

 Hinge-line straight, a little less than the length of the valve, and 

 greater than the height. Beak sub-anterior, directed forward, acute, 



