102 
NATURAL HISTORY. [NEW BUILDING. 
are short and rounded* and sometimes serrated or torn be¬ 
neath. As the animals of the different genera are so simi¬ 
lar* it is necessary to divide them into sections* according to 
the form of the shell. The first section* which have been 
generally called Snails* have the whorls twisted round a 
short axis into a subglobose shell* with a crescent-shaped 
mouth, formed by the projection of the last whorl but 
one into its cavity. The animal has a distinct and 
variously divided vesicula multifida. The true Helices * 
Helicodonta * &c., have the peristoma of the shell thick¬ 
ened* while the Helicophanta * EpistpUum, and Proser¬ 
pina have it thin and sharp. The second group chiefly 
differ in the whorls being on a longer axis* so that the shell 
is oblong. These animals have no vesicula multifida, and 
the mouth of the shell is longer than it is broad. This 
group is again subdivided into those that have the mouth of 
the shell continued without interruption into the pillar lip. 
The axis of their shell is generally perforated, especially 
in the young state, for the animal, as it grows, sometimes 
covers the perforation with the reflexed portion of the inner 
lip; and the eyes are placed* as in the snails* on the tips of 
the blunt tentacles. The genera of this section, as in the 
snails* is distinguished by the modifications that the mouth 
of the shell assumes when the animal has arrived at its per¬ 
fect state ; for during their growth they all have the same 
thin, simple lips. Thus the Bulimi have simple* thick¬ 
ened* reflexed lips* and gradually enlarging* whorls. 
The Pupce have one or more solid teeth formed by the 
thickening of the inner edge of the lips, and the 
whorls enlarge in diameter so gradually (after the very 
early age of the animal) :hat the shell generally assumes 
a cylindrical form. The Chondri chiefly differ from the 
former in the mouth being armed with long plaits* formed 
by the inflection of the surface of the shell when the 
animal is about to complete its mouth, which forms ex¬ 
ternal grooves and internal ridges. The Clausilice are 
similar to the latter* but have a continuous groove in front 
of the last whorls* and the animal forms* a short time be¬ 
fore it arrives at the adult age and is about to complete 
the mouth of the shell* an expanded plate* (which is evi¬ 
dently a peculiar modification of a tooth*) attached to the 
pillar of the shell by a slender pedicel* and placed in such 
