Lamellibranchiata. 357 



Formation and localities. In sandstones of the Upper Chemung 

 group, McKean county, Pa., and Farmers' Valley, Cattaraugus county, 

 N. Y. 



Leptodesma sociale. 



Leptodesma sociale, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 

 PI. 21, figs. 24-28 (33, 34?). Jan., 1883. 



Shell small, sub-rhomboidal ; body broad-ovate, oblique ; length 

 greater than the height ; anterior margin nearly straight and quite 

 oblique ; base regularly rounded ; post-basal side somewhat extended 

 and abruptly recurved. Left valve convex, gibbous in the upper 

 part, more convex than the right valve, which is moderately con- 

 vex above and depressed-convex or flat below. Hinge-line straight, 

 sometimes greater than the length of the shell. Beaks at about the 

 anterior third of the hinge-line ; the left beak prominent, obtuse ; 

 the right beak less conspicuous. Umbonal angle acute. Anterior 

 extremity sub-auriculate, straight above, slightly extended. Wing 

 triangular, joining the body of the valve below the middle of its 

 length (measured from hinge to base) ; margin regularly concave ; 

 extremity produced into a spine, which, in perfect specimens, reaches 

 beyond the margin of the valve. In the right valve the wing is less 

 distinctly defined, and the sinus not so deeply concave. 



Surface marked by fine concentric striae, which at somewhat 

 regular intervals are elevated into sharp, conspicuous lamellae, or 

 sometimes into fascicles of striae producing elevations. These be- 

 come crowded and irregular on the anterior of the valve, and are 

 very regular and uniform on the wing and on the right valve. 



A specimen of a left valve has a length of 10 mm., height 7.5 mm., 

 hinge-line about 10 mm. A similar specimen has a length of 11 

 mm., height 6.5 mm., and hinge-line about 11 mm. A right valve 

 has a length of 9 mm., height 6 mm., hinge-line about 8 mm. 

 Another one has a length of 16 mm., height 11 mm., hinge-line 

 about 16 mm. 



This is a small and abundant species, often occurring in great num- 

 bers on the surfaces of shaly layers. It is distinguished by its form 

 and the regular lamellose appearance of the surface. 



Formation and localities. In the shales of the Chemung group at 

 Conewango, Cattaraugus county, Nanticoke Springs, Broome county, 

 and south of Ithaca, N. Y. 



