ROOM XIII.] NATURAL HISTORY. 93 
are retractile into themselves, like the linger of a glove, as 
in the family of the Slugs ( Limacidce ), Case 55 ; which 
have either only a few calcareous grains in the mantle, as in 
the genus Avion; or a small shell imbedded in this part, 
as in the Limax ; or a small ear-shaped shell placed over 
the mantle at the end of the body, as in Testacella . The 
family of Snails ( Helicidce ) have a large shell, into which 
the animal can withdraw itself, as in the genera Helix , 
Bulimus , Partula , Pupa , Vertigo, and Claucilia. These have 
the edge of the mouth thickened, and often toothed ; others, 
as Succinia , Achatina , Zonites, Namnia, and Vitrina, have 
the edge of the mouth thin: the last two have the front 
of the mantle produced beyond the mouth of the shell, 
like a shield, and a lobe at its side, which partly covers 
the shell, and gives it the polished coat for which they are 
peculiar. 
Those which live in water, have subulate, contractile 
tentacula, with the eyes on their inner base, as in the family 
of AuricuVidcje , containing the genera Auricula and Me- 
lampus , and others, have their eyes near their outer base, 
as in the family of Pond Snails , ( Lymnceadce ). Some of 
these, as the Lymn&a, have the inner lip of the shell 
plaited; while the others as the Physce and Planorbes, 
have the shell constantly reversed, or with the whorls 
turning from the left to the right, contrary to their direc¬ 
tion in most other shells ;—and the Ancyli, which have 
simple conical shells, having the tip bending in the same 
direction. Many of these animals, during their torpidity, 
cover the mouths of their shells with a membranaceous or 
calcareous case, which is dissolved or thrown off when 
they revive. 
The family of the Cyclostomidce differ from all the other 
Land Mollysca, in having the respiratory cavity open in 
front, and in not being hermaphrodite: they have subulate 
contractile tentacula, with the eyes at the base, like the 
pond-snails. They are the only land shells which have an 
operculum ; this family contains the genera Cyclostoma 
and Helicina. 
The Cases 66 to 86 contain the second class of Mollusca, 
which have bivalve shells, and whose animals are always 
covered with a two-lobed mantle, each protected by a 
shelly valve, and they have within the mantle, between it 
