Lamellibranchiata. 377 



broadly rounded. Left valve regularly convex below, narrow and 

 gibbous above. Eight valve depressed-convex below, more convex 

 above, and moderately gibbous in the umbonal region. Hinge-line 

 straight, somewhat less than the greatest length of the body. 

 Beaks sub-anterior, acute, directed forward, slightly elevated above 

 the hinge-line. Umbonal region abruptly gibbous, subtending a 

 very acute angle. Anterior end short. Wing joining the body near 

 the posterior extremity; margin regularly concave, the greatest 

 concavity near the middle of the width, terminating in a short 

 spiniform extension. In the right valve the wing is proportionally 

 larger and less defined. 



Test thick, marked upon the body and wing with sharp, elevated, 

 lamelliform striae and intermediate finer striae; on the anterior the 

 striae are fasciculate. 



A large left valve has a length of 40 mm., height 26 mm., hinge- 

 line 32 mm. A right valve has a length of 36 mm., height 20 mm., 

 and hinge-line 34 mm. 



This species resembles L. Hector in general form, but is broader at 

 the posterior end, more abruptly gibbous along the axis above the 

 middle, the wing joins the body nearer the posterior extremity, and 

 the margin of the wing has the deepest concavity in the center of its 

 width. The right valve is more convex than the right valve of L. 

 Hector, and the concentric elevated striae are much stronger. In all 

 these characters it also differs more extremely from L.extenuatum. 



Formation and locality. In compact sandstone of the Upper Che- 

 mung group, alternating with some red beds, on Seely creek, Tioga 

 county, Pa. 



Leptodesma Pelops. 



Leptodesma Pelops, HaijIj. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell larger than the medium size, sub-rhomboidal ; body ovate, 

 oblique at an angle of about 40 p with the hinge-line; height two- 

 thirds the greatest length; ante-byssal and ventral margins very 

 oblique and continuing in nearly a straight line for two-thirds the 

 length, with a slight concavity for the byssal sinus ; posterior mar- 

 gin very broadly curving. Left valve depressed-convex below, 

 gradually becoming more convex above the middle, and somewhat 

 gibbous on the umbo. Right valve unknown. Hinge-line straight, 

 nearly equal to the greatest length of the shell. Beak sub-anterior, in- 

 clined forward, obtuse, scarcely elevated above the hinge-line. Um- 

 bonal angle acute. Anterior end triangular, prolonged, acute. Wing 

 [Sen. Doc. No. 38.] 48 



