LAMELLIBEANCHIATA FEOM THE EOCENE MAKLS. 225 



Mv. Conrad's description seems to have been made from a cast of a 

 small left valve, on which the radii were less numerous than usual. Among 

 the casts and impressions whicli I have examined from these beds in New 

 Jersey I find no evidence of more than one species, although if the differ- 

 ence in number of ribs be taken as evidence of specific characters, there 

 might easily be two or even three species founded upon those which 

 are now before me, as one individual, an external imprint, half an inch long, 

 shows twenty-nine ribs on the body of the shell, while another, also an ex- 

 ternal imprint, onlj^ two-thirds as large, shows thirty-eight. The larger 

 right valve figured, which is from a gutta-percha impression of an external 

 imprint, shows fifty ribs when those on the submargins are counted. An 

 internal cast of a left valve of nearly equal size to the last shows only 

 about twenty-five ribs which are less distinctly elevated, being low and 

 flattened, those on the submargins being lost, while all those added intersti- 

 tially on the exterior are lost on the internal cast. 



The shell, as I find it represented by casts of the interior and imprints 

 of the exterior, is subdiscoidal, or suborbicular in general outline, with 

 moderately and generally evenl}' convex valves, and moderately sized ears, 

 the anterior one on the right valve being slightly longer than the posterior, 

 distinctly separated from the body of the shell b}- a deep, narrow sinus; 

 posterior ear rapidly sloping from the hinge line and joining the body of 

 the shell below much farther from the central line than its length on the 

 hinge line. Hinge line straight on both valves and about half as long as 

 the antero-posterior length across the body of the shell below. The left 

 valve has ears about equal to those of the right valve, but without the an 

 terior sinus. Body of the shell abruptly and distinctly elevated above the 

 plane of the auriculations along the submarginal slopes Radii sharplv 

 elevated, rounded on top, and separated by equall)' sharp, rounded de- 

 pressions, increased by implantation, but not frequently enough to prevent 

 this rapid increase in strength as the shell increases in size, numbering 

 from fifteen on young shells to fifty on larger individuals Surface marked 

 by concentric striae, very fine and even on the body of the shell, but 

 stronger, more elevated, and more distant on the lateral portions of the 

 shell. Ribs stronger in the middle of the shell and finer on the sides. 

 Auriculations marked by rays and strong concentric striae. 

 4418 MON 9 1.5 



