90 
NATURAL HISTORY. [^UPPER FLOOR, 
Tlie tentacula of the kinds which always live on land, 
are retractile into themselves, like the finger 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 HeliXj 
Bulimus, Partula, Pupa, Vertigo, and Clausilia, 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 Aui'iculidce, containing the genera Auricula and Me- 
lampus, and others, have their eyes near their outer base, 
as in the family of Pond Snails, (^Lijmnceadce). Some of 
these, as the Lymncea, have the inner lip of the shell 
plaited; while 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 direction in 
most other shells;—the Ancyli have simple conical shells, 
with 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 Mollusca, 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 
