244 Annals of the Carnegie Museum. 



usually occurs as internal casts in sandstone, and as the present col- 

 lection does not contain any specimens showing the exterior, Billings' 

 original description is here repeated. 



Description. 



"Subquadrate, large, very gibbous, hinge line less than the greatest 

 width of the shell; front margin gently convex, straight, or slightly 

 concave; front angles rounded; a portion of the sides, ecjual to about 

 one third the length along the middle, usually straight, but sometimes 

 a little convex; above the sides curve inward to the cardinal angles. 



"The ventral valve is moderately and somewhat irregularly convex, 

 the beak small, pointed, and much elevated. In most of the speci- 

 mens a broad, low mesial ridge or depressed fold extends from the beak 

 to the front margin; sometimes the fold is barely perceptible or ob- 

 solete, and in such cases the whole of the valve, except a small space 

 at the cardinal angles is flat, and slopes with scarcely any curvature 

 from the beak to the front; at each of the cardinal angles a portion of 

 the shell is depressed toward the dorsal valve. Area large, triangular, 

 a little arched. Foramen not large, extending nearly to the apex of 

 the beak. 



"Dorsal valve very convex, most elevated in the upper half, and 

 sloping abruptly to the sides, front and cardinal angles, the latter 

 strongly reflected, as is also in some specimens a narrow border along 

 the upper third of the sides; the beak and area strongly incurved 

 over the hinge line. Along the middle of this valve a broad, shallow 

 mesial furrow extends from the beak to the front. The foramen is 

 occupied by a sharp cardinal process. 



"The surface is covered Avith moderately coarse, radiating ridges, 

 about four or five in the width of two lines, at the margin. They appear 

 to be two or three times subdivided between the beak and the front." 



The interior of the brachial valve shows small dental sockets, bounded 

 by lamellae which are supported by branches of the median septum. 

 The delthyrium is wide and open, the cardinal process small and linear. 

 The muscle scars are not deeply impressed, but on some specimens two 

 pairs of adductor scars can be made out. 



In the pedicle valve the muscle area is very strongly marked, some- 

 what triangular in outline, and widest at the front. At the sides it is 

 bounded by strong lamellae which support the teeth. At the front 

 of the scar a sort of platform is formed by the thickening of the shell. 



