101 
EAST. ZOOL. GAL.] NATURAL HISTORY. 
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 . 
2. The family of Snails, Helicidce , (Case 23—25,) 
have a tapering tail, without any terminal gland. Their 
pulmonary cavity is generally 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 con¬ 
tains many species and genera which are difficult to dis¬ 
tinguish, 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 
toot the whole length ; among these the Limacellus and 
Mehimatium are peculiar for having a large mantle cover¬ 
ing the whole of the back, while 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 mem¬ 
branaceous mantle, with a thickened margin, which incloses 
the protruded bag that contains the 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 ex¬ 
ternal shell. Among the genera of this group, which in¬ 
cludes a very large proportion of the species of the order, 
the Testacella are 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 cylindri¬ 
cal 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 
