72 
NATURAL HISTORY. [UPPER FLOOR. 
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. This division 
includes the genera Naiica, Littorina, Odo?itosto?na, Tur - 
ritella, Scalar ia, Eulima , Rissoa , Cerithium , and Trio stoma ^ 
all of which live in the sea, or at least in brackish water, 
and Neritma and Valvata, which are found in rivulets ; 
the latter is peculiar for its branchiae being protruded be¬ 
yond 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 formed of con¬ 
centric rings. The young of the former are born alive, 
being then covered with bands of cilia. Then follow the 
FooFscap Limpet, ( Capulus ,) which is attached, by the back 
of its foot, to shells and other marine bodies, on which it 
forms a smooth disc, either by dissolving the surface, or by 
depositing on it a shelly plate. Lastly, succeed the genera 
Velutina , Crepidula , and Calyptrea which have no opercula. 
The branchiae of the animals of the last two genera are 
formed of long filiform processes, placed at a very oblique 
angle across the back of the neck. 
In the end of the Table are placed the shells of animals 
which greatly resemble the former in the form of their 
branchiae, but differ in having 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. And lastly the genus Lottia , which 
