LAMELLIBRANCHIA TA. 
187 
This species is distinguished from L. Rogersi and L. spinigerum by the large 
anterior end, narrower umbo, more extended wing, with less deeply sinuate 
margin, less convex right valve, and the regular concentric lamellose ridges 
of the surface. 
Formation and localities. In sandstones of the Upper Chemung group, McKean 
county, Pa.; and Farmer’s Valley, Cattaraugus county, N. Y. 
Leptodesma sociale. 
PLATE XXI, FIGS. 24-28 (33, 34 ?). 
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 regu¬ 
larly 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 convex 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 promi¬ 
nent, 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 become crowded and irregular 
on the anterior of the valve, and are very regular and uniform on the wing 
and on the right valve. 
Interior unknown. 
