68 
NATURAL HISTORY. 
[EAST. ZOOL. 
the air hole is in the front of the edge of the mantle, and the orifices 
of reproduction are near this aperture. The shells of the animals of 
this family are in very different degrees of developement. Sometimes, 
as in Avion , when the body is simply elongated, it consists of only a 
few calcareous grains; in others, which have the digestive and more 
important organs as it were protruding from the body in a small bag-like 
mantle, this part is covered with a spiral shell, which is generally thin, 
and sometimes itself covered, (when it is polished,) with some reflexed 
lobes of the mantles, as in Helicarion, or nearly naked, with only a 
thick edge to the mantle of the animal, as in Nanina and Stenopus. 
The family of Snails (HELiciDiE, Cases 23—25) have a taper¬ 
ing tail, without any terminal gland. Their pulmonary cavity is gene¬ 
rally on the front of the back, and the respiratory hole is in the 
hinder part of its margin, while the apertures of the reproductive organs 
are near the base of the right tentacle. This family contains many 
species and genera which are difficult to distinguish, from the gradual 
manner in which they pass into each other.—A few genera are destitute 
of any shell, or if they have any, it is quite hidden in the mantle ; their 
body is always elongate and attached to the foot the whole length; among 
these the Limacellus and Mehimatium are peculiar for having a large 
mantle covering the whole of the back, wffiile the true Slugs, Limax , 
have only a shield-like mantle on the front of the back, like the Arions. 
Most of the genera have a more or less exposed shell, which is placed 
on and protects a thin membranaceous mantle, with a thickened margin, 
which incloses the protruded bag that contains the most important 
internal organs ; in Parmacella , Vitrina , Helicolimax, &c., the margin 
of the mantle is broad, compared to the size of the partly inclosed shell, 
and forms a shield on the front of the body; in Plectophorus it is 
divided into two parts, the front part being shield-shaped, and the hinder 
elongate, bearing an external conical shell. In the remaining genera 
of this family the mantle is simple, thin, and covered with an external 
shell. Among the genera of this group, which includes a very large 
proportion of the species of the order, the Testacella is peculiar for 
having the mantle and the ear-shaped shell on the posterior extremity 
of the body. This animal also has the power of extending the edge of 
the mantle, so that it can cover up the whole of the contracted body, 
and thus protect it from drought; its lips are cylindrical and retractile, 
like the tentacles : they live, the greater part of their time, in holes 
under the ground, where they feed on earth-worms. In all the other 
genera the mantle and shell are on the central part of the foot, and the 
lips are short and rounded, and sometimes serrated or torn beneath. 
As the animals of the different genera are so similar, it is necessary to 
divide them into sections, according to the form of the shell. The 
first section, which have been generally called Snails, have the whorls 
twisted round a short axis into a subglobose shell, with a crescent¬ 
shaped mouth, formed by the projection of the last whorl but one into 
its cavity. The animal has a distinct and variously divided vesicula 
multifida. The true. Helices , Helicodonta, &c., have the peristoma of 
the shell thickened, while the Helicophanta , Epistylium , and Proser¬ 
pina have it thin and sharp. The Streptaxis is peculiar for the animal 
when it arrives at a certain period of its life suddenly altering the 
direction of the whorls of the shell, so as to move it out of the axis of the 
