NATURAL HISTORY. 
71 
GALLERY. ] 
the animal is expanded. The three first genera have an upper and two 
lateral jaws, and simple conical teeth on the tongue, while the two latter 
have only a single upper jaw and serrated teeth. The Ancyli only differ 
from the Pond Snails in having a simple conical shell, which has caused 
them to be called fresh-water Limpets. The Velletice differ in the 
animal and shell being reversed, like the Physce. The animals of Seg¬ 
menting which are like the Planorbes, deposit at each stoppage in their 
growth three transverse plates, which contract the mouth of the shell, 
and make it appear as if it was chambered; hence it has been called a 
fresh-water Nautilus ; and Discus has teeth in the cavity of the shell. 
C. The marine lung-breathing Mollusca have no distinct tentacles, 
as the three following families:— 
The family Amphibolid^e (Case 26) have a subglobose, spiral, 
umbilicated shell, with an oval mouth and a rather expanded outer lip, 
which is sinuated behind ; the head has no tentacles, but a transverse 
disk across the front, which is free at top, and has the eyes placed on its 
outer hinder side. The operculum is horny and spiral. They are only 
found in the Pacific Ocean. 
The family of Siphonariad^e (Case 26) are like the former, but 
they are protected by a conical shell like a Patella , with which they 
have been confounded; but the apex is rather on one side and posterior, 
and the scar left by the adductor muscle near the edge of the cavity (as 
in Ancylus) is interrupted in the centre of the right side, where the 
breathing-hole of the dorsal respiratory cavity is placed; they have no 
operculum. 
The family of Gadiniadje (Case 26) 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 CyclostomiDxE (Case 26) have a spiral oper¬ 
culum, 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 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 w 7 alks by alternately moving forward 
first one and then the other of these sides. In Cyclophorus , Ptero - 
cyclos and Megalomastoma , w 7 hich have a horny many-whorled oper¬ 
culum, the foot is simple, and the animal glides along like most other 
Gasteropodes. Some of these, as Pterocyclos, have a more or less de¬ 
veloped groove or hole at the hinder angle of the mouth, evidently 
formed by some periodically developed process of the edge of tne 
mantle. The operculum of this genus, like that of Siliquaria , 
after it has arrived at its full size, continues to form new whorls, 
