CHONETES OF THE HAMILTON GROUP. 127 
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. 
Sukface of the ventral valve marked by from twenty-six to thirty or 
thirty-four subangular or sometimes rounded striae, 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 striae 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-lateral 
margins nearly flat. The surface is finely pustulose in the middle, a little 
