COMPOSITA 491 



Surface of the shell marked only by concentric lines of growth which 

 are sometimes stronger and more crowded towards the outer margin of 

 the valves. 



Remarks. This species differs from C. siibquadrata, its nearest ally, in 

 its much deeper sinus towards the front, a character which is more 

 strongly developed in this species than in any other member of the genus 

 here defined. 



Horizon. Chester group. 



COMPOSITA L^VIS n. sp. 

 Plate LXXXII, Figs. 14-20 



Description. Shell small, longer than wide, with the greatest width 

 anterior to the middle, subovate in outline, with the anterior margin 

 emarginate. The dimensions of two. nearly perfect examples are: length 

 of pedicle valve 12 mm. and 10.7 mm., length of brachial valve 11 mm. 

 and 10 mm., greatest width 10.3 mm, and 9.3 mm., thickness 7.5 mm. and 

 6.5 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly to the postero-lateral margins and strongly inflected to the car- 

 dinal extremities, the curvature much more gentle to the antero-lateral 

 and anterior margins; mesial sinus short, originating near or a little in 

 front of the middle, either remaining shallow or attaining a moderate 

 width and depth at the anterior margin; the beak attenuate and mod- 

 erately incurved, coming nearly in contact with the umbo of the opposite 

 valve, truncated obliquely to the plane of the valve, the foramen rather 

 large, elliptical in outline, encroaching entirely upon the umbonal por- 

 tion of the valve, the delthyrium filled with the beak of the opposite 

 valve. 



Brachial valve subequally or a little more convex than the pedicle, the 

 greatest convexity posterior to the middle, the umbo prominent and 

 projecting notably posteriorly beyond the hinge-line, the surface curving 

 abruptly to the postero-lateral margins and becoming somewhat inflected 

 at the cardinal extremities, curving more gently to the antero-lateral 

 and anterior margins; mesial portion of the valve somewhat flattened 

 anteriorly from the middle, usually becoming slightly depressed in a 

 short and shallow median sinus near the anterior margin, which, meeting 

 the sinus of the opposite valve, produces the emargination of the shell 

 in front ; beak sharply pointed and strongly incurved beneath that of 

 the opposite valve. 



Surface of both valves nearly smooth, marked only by a few obscure, 

 concentric lines of growth which are commonly present only towards 

 the anterior margin. 



