MOLLUSCA. 185 
. Cavity of the shell interrupted by a testaceous plate. 
This division consists of five genera, each of which may 
be regarded as the type of a family, although, for the pre- 
sent, they are all included in one. In the Crepidula, the 
gills form a transverse ridge on the roof of the cavity, con- 
sisting of filaments extending beyond the margin. The eyes 
are at the base of the tentacula. There is only one fluvi- 
atile genus, termed Navicella. — 
B. Cavity of the shell entire. 
In the Capulus, the shell is entire, the foot is complicated 
on its anterior margin. The shell adheres to the animal by 
a circular muscle, leaving an opening in front, for the issue 
of the head and entrance to the branchial cavity. The gills 
form a single ridge across the roof. The mouth is in the 
form of an extended proboscis, with a deep groove above. 
The tentacula, which are two in number, have the eyes at 
the external base. The anus is on the right side of the 
branchial cavity. In the Carinaria, the foot appears to be 
compressed, and formed for swimming. The head is co- 
vered with a group of tubercles. The mouth is furnished 
with a proboscis. Near the middle of the body the shell is 
attached. The surface of the body above is closely covered 
with small tubercles. It is probable that the species here 
alluded to is the same with the Pterotrachea coronata ot 
Forskal. 
Fissurellade. Shell with a slit or perforation. In the 
Fissurella, the apex of the shell is perforated, and united to 
the cloak by a circular muscle open in front. The cloak 
forms a duplicature in front for the branchial cavity, which 
extends to the perforated apex of the shell. The gills con- 
sist of two ridges ; at the dorsal extremity of which is the 
anus. It is probable that the excrements are ejected at the 
