LAMELLIBRANCHIATA. 187 



This species is distinguished from L. Rogersi and L. spinigerum by the large 

 anterior end, narrower undio, 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. Sl-M (33, 34 ?). 



Lfptodesma sociale, Hai.l. Pal. N. Y.. vol. v. pt. 1. Plates an<l Explanation.s : PI. 21. tig-s. 24-2S (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 lielow. 



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 suli-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 jjerfect specimens, reaches beyond the 

 margin of tlie valve. In the right valve the wing is less distinctly defined, 

 and the sinus not so deeply concave. 



Surface marked by fine concentric stria?, which at somewhat regular 

 intervals are elevated into sharp, conspicuous lamellaj, or sometimes into 

 fascicles of strige 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. 



