GALLERY.] 
NATURAL HISTORY. 
133 
The family of Gadiniadcz have an animal and shell like 
the former, but the passage to the respiratory cavity is in 
front of the adductor muscle, its place being marked in 
the cavity of the shell by a slight groove. 
II. In the remainder of the animals of this order, the 
respiratory chamber is open, that is to say, the front edge 
of the mantle is free from the back of the neck, leaving a 
large slit for the admission of the air into the bag. They 
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 woods, and under stones. 
The family of Cyclostomidce 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 fur¬ 
nished with an external rim. The operculum is some¬ 
times extremely beautiful, having a more or less developed 
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 , Piero - 
cyclos and Megalomastoma, which have a horny many- 
whorled operculum, the foot is simple, and the animal 
glides along like the 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- 
stoma 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 Pomatias have an elongated shell with re 4 
flexed lips, and a horny oval operculum. 
