106 
NATURAL HISTORY. [NEW BUILDING. 
are unisexual, have a distinct operculum, and two rather 
elongated contractile tentacles, with the eyes at the outer 
side of their base. They are all terrestrial, living in 
damp places in w T oods, and under stones. 
10. The family of Cyclostomidce (Case 26) have a spiral 
operculum, and the edge of the mantle of the animal is 
thin and simple. The mouth of the shell is round, and 
often furnished with an external rim. The operculum is 
sometimes extremely beautiful, having a more or less deve¬ 
loped raised edge to the outer side of each of the whorls, 
in the Cyclostoma , which have a shelly operculum and 
simple mouth to the shell, the foot of the animal is divided 
into two equal parts by a longitudinal groove, and the 
animal walks by alternately moving forward first one 
and then the other of these sides. In Cyclophorus , Ptero - 
cyclos and Megalomastoma, which have a horny many- 
whorled operculum, the foot is simple, and the animal 
glides along like most other Gasteropodes. Some of these, 
as Pterocyclos, have a more or less developed groove or 
hole at the hinder angle of the mouth, evidently formed 
by some periodically developed process of the edge of the 
mantle. In others, as Megalomastoma , there is a perma¬ 
nent appendage to the mantle, which produces a groove 
and ridge in the front of the mouth near the pillar; and 
in Pupina , which has been confounded with the Buccina, 
the groove ends in a narrow-edged marginal notch. The 
CallicE have a peculiarly polished shell very like the former, 
but they want the groove. The fossil genus Stropho- 
sioma is peculiar for the animal, when it is approaching to 
its adult state, suddenly reversing the position of its body, 
so that the mouth of the shell is placed on a plane with the 
surface of the spire, as in the genus Anostoma among the 
Helices. The Pomaiias have an elongated shell with re- 
hexed lips, and a horny oval operculum. 
11. The family of Helicinas {Helicinidce , Case 26) have 
a half ovate annular operculum, and the edge of their 
mantle is said to be thickened like that of the snails. The 
mouth of the shell is half ovate, with a reflexed edge : they 
use their elongated tapering tentacles to feel their way as 
they walk. The shell of the Helicince have a simple 
mouth. Alcadia differs in having a slit in front of the 
mouth, into which is fitted the tooth-like process o£#pe 
