128 
NATURAL HISTORY. [EAST. ZOOL. 
of the body to appear truncated. Their respiratory ca¬ 
vity is on the anterior part of the body, 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 
Sten opus. 
The family of Snails, Helicidce, have a tapering tail, with¬ 
out 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 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 
Mekimatium are peculiar for having a large mantle cover¬ 
ing the whole of the back, while the true Slugs, Limaoc s 
have only a shield-like mantle on the front of the back, 
like the Arions . Most of the genera have a more <5r less 
exposed shell, which is placed on and protects a thin mem¬ 
branaceous mantle, with a thickened margin, wdiich incloses 
the protruded bag that contains the internal organs. In 
Parmacella , Vitrina, HelicoUmax, &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. 
