BOOM XII.] NATURAL HISTORY. 69 
mantle, which, as the animal grows towards maturity, is 
spread out, so as to cover the back of the shell with two 
large lobes. These lobes the animal is capable of contract¬ 
ing into the cavity of the shell at will, by which it differs 
from the genus Coriocella, the latter having a thin earlike 
shell, which is embedded in the back of the mantle of the 
animal. 
Cases 3 and 4 contain the shells of those Gasteropodous 
Mollusca, that have the branchiae similar to the former, 
but no syphon on the front of the mantle, and consequently 
no canal in front of the shell. Many of them have a spiral 
operculum or lid, which is attached to the back of the 
hinder part of the animal: this operculum turns round on 
the apex of its spire as it increases in size. 
Some have the eyes placed on short pedicles at the back 
of the tentacula, and the heart surrounding the rectum. 
They generally have a fringe on each side of their body, as 
the genera Trochus, Monodonta, and Haliotis. These have 
a pearly appearance on the inner surface: the two former 
are furnished with spiral opercula, and the latter is provided 
with a series of holes, by which the water is introduced into 
the branchiae. The genera Fissurella and Emarginula, 
are not pearly, but the former has a hole near the apex, 
and the other in the front margin of the shell, for the 
passage of the water to the branchiae, and the expulsion of 
the faeces. The Neritina are not pearly, and have no 
fringe, they have the operculum articulated to the pillar 
lip. Lastly the genus Lotlia has a shell exactly like 
Patella in shape, while the animal very nearly resembles 
those of the two latter genera; except that it has only one 
btanchia placed obliquely across the back of the neck, 
which is exerted when the animal walks. 
Others have the eyes sessile at the base of the 
tentacula, and the heart separate from the rectum. This 
division includes the genera Natica , Littorina, Odon - 
tostoma , Turritella , Scalaria, Eulima, Rissoa, Ceri- 
thium, and Triostoma, all of which live in the sea, 
or at least in brackish water, and Valvata , which is found 
in rivulets, and is peculiar for its branchiae being pro¬ 
truded beyond the shell when the animal walks, and 
formed of spiral plates. The other genera which live also 
in rivers, as Paludina and Bithinia , have the opercula 
