168 DESCRIPTION OF SHELLS. 
Cardium Hippopeum. 
A rare shell at Bracklesham, of which only fragments have been discovered. 
It is important as marking the close relationship of the Bracklesham beds with 
those of Chaumont, Parnes, &c. of France. 
Cardium semigranulatum. (Tab. IL. fig. 20.) 
Barton, London, and Bracklesham Bay have each their peculiar variety of this 
shell. The granulated portion in the Barton variety is gradually rounded into 
the smooth part of the surface, and forms a very wide angle on the margin, so 
that the outline is nearly circular ; that portion of the London shell, on the other 
hand, is flattened, and forms a ridge at its junction with the rest of the surface ; 
its edge is also nearly straight or rather concave, and the base line being but 
little curved the shell has a rhomboidal contour. The Bracklesham shell is 
between the two, and has the granulated surface convex, but not so gradually 
rounded into the smooth part as in the London variety ; on the margin it forms 
aright angle, and the base line being very much curved the outline is irregularly 
orbicular. 
Specimens often occur at Bracklesham considerably larger than the figure, 
but the largest are found at Barton. 
Cardium porulosum. 
The variety found at Bracklesham is that without pores through the elevated 
lamin ; the specimens are frequently large ; the Barton variety has pores. 
Cardium alternatum. (Tab. III. fig. 14.) 
Spec. Cuar —Cordiform, posteriorly truncate, surface covered by forty or 
more smooth ribs ; those on the posterior area broad, ornamented with V-shaped 
plates whose angles are directed towards the beak ; on the middle portion of the 
surface the ribs are narrow, alternately furnished with thick lunate scales ; on 
the anterior portion nearly all the ribs are naked. 
This beautiful little shell, which much resembles in form the common Cockle, 
is seldom above half an inch long, and is very rarely found perfect. The ribs are 
very regular, with deep narrow channels between them; those on the posterior 
portion are concave in the middle, the others round. The scales upon them 
are thick, regular, and nearer together than they are long ; on the central portion 
