CHONBTES OF THE HAMILTON GROUP. laT 



closed by a pseudo-deltidium, and the aperture occupied by the cardi- 

 nal process of the opposite valve. Dorsal area more than half as wide 

 as the ventral, and marked in the middle by a wide triangular 

 callosity. 

 Surface of the ventral valve marked by from twenty-six to thirty or 

 thirty-four subangular or sometimes rounded stria3, which are often 

 irregularly increased by bifurcation or intercalation towards the 

 margin. In those with fewer striae, they are sharper, more abruptly 

 elevated, and only half as wide as the interspaces ; while in those 

 with a larger number, the striae and interspaces are equal, but some- 

 times the striae become fuller and more rounded, and the interspaces 

 proportionally less in width. The striae on the dorsal valve correspond 

 essentially with those of the ventral valve, and there is a considerable 

 space at the cardinal angles of each valve destitute of striae. Fine 

 closely arranged concentric striae are visible on the surface of well- 

 • preserved specimens. 



The interior of the dorsal valve shows a slender elongate cardinal 

 process which is scarcely bifid at the extremity, and has on each side, a 

 little below the apex, a minute lateral process for muscular attachment. 

 The dental sockets are limited on the upper side by a narrow ridge, and 

 on the lower side by a stronger oblique ridge which supports the base of 

 the cardinal process. The two pairs of occlusor muscular imprints are 

 distinctly marked; and outside of these, the vascular impressions are 

 pretty well defined. Beyond the vascular impressions the surface is 

 covered by elongate papillae, the marks of the strias being scarcely 

 distinct. 



The interior of the ventral valve shows strong dental lamellae ; a some- 

 what broad and angular median ridge terminates above the middle of the 

 valve. The occlusor muscular impressions have not been observed ; and 

 those of the divaricator muscles are wide and spreading, but not dis- 

 tinctly defined. The cavity of the shell is abruptly rounded below, and 

 the shell abruptly deflected at the sides, leaving the cardino-laterai 

 margins nearly flat. The surface ia finely pustulose in the middle, a little 



