92 
NATURAL HISTORY. [NEW BUILDING. 
a free oval, spiral, or subspiral horny operculum ; the shells 
generally have a roundish mouth, and are not pearly within; 
some have a simple round mouth, with a concave inner lip, 
as the Littorince and Assiminia. Lacuna differs in the pillar 
lip being flattened, and the axis pierced; Seal-aria is re¬ 
markable for having a white shell, marked with numerous 
varices, and the whorls are often only united to one an¬ 
other by the projections of the varices, thus exhibiting 
what is the case with all shells, that they are only formed 
of a coiled up, gradually enlarging tube; the Turritellce 
are turreted, and the Melanice and Cerithia, which have 
also generally elongated turreted shells, usually have a 
more or less distinct groove or nick in the front of the 
mouth. The Telescopium and Pyrazi have an orbicular 
many-whorled operculum, and the Cerithia an ovate one. 
Some, as the Melanopses, have a nick quite like the 
Buccina; but they are easily known from the marine 
zoophaga by their structure and mode of life, which is 
passed in fresh water; their apices are generally eroded, 
and they are often covered with a dark olive periostraca. 
The Tristoma are peculiar for having a contracted mouth, 
and a tubular hole, like the Typhis , at the posterior angle 
of the outer lip. 
The family of Looping Snails ( Truncatellidce , Case 20) 
have an oval spiral operculum, and the same form of ani¬ 
mal as the former family, but the foot is small, and the lips 
very large and expanded ; the animal walks by contracting 
the space between these parts into a loop, like the Looper or 
Geometric Caterpillars. 
The family of Pond Snails, ( Paludinidce , Case 21,) like 
the Ampullariadce , and unlike the other families of this 
group, are fresh water Mollusca, with the opercula formed 
of regular rings, but they differ from the Ampullariadce in 
having short tentacles, sessile eyes, and an entire forehead. 
The Paludince have a horny operculum, and are ovovivipa- 
rous, while the Bit hi nice, which have a shelly operculum, 
are oviparous like Lymncea . 
The family of Velutinidas (Case 21) have an animal 
very like the Littorinidce, but they are destitute of any 
operculum, and the shell is small for the size of the animal, 
with a very short spire and a large mouth. They live 
on stones on the shore. 
