81 
differences are not so striking. The cowries ( Cyprcea ), 
and the Tear-Shell {Erato), are remarkable for the 
young shells being very thin and exposed, whereas the 
adults are covered with a thick coat, deposited over 
their back by the 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 contracting 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 Gastero- 
podous 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 attach¬ 
ed 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 
Natica , Littorina, Odontostoma, Turritella, Scalaria, 
Eulima , Rissoa , Cerithium, and Triostoma , all of 
which live in the sea, or at least in brackish water, and 
Neritina and Valvata, which are found in rivulets; 
the latter is peculiar for its branchiae being protruded 
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 formed of concentric rings. The young of 
the latter are born alive, being then covered with 
bands of cilia. Then follow the Fool’scap Limpet, 
Eapulus , 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 pro- 
g cesses, 
ROOM XII. 
Nat. Hist. 
