330 
NATURAL HISTORY. [\f2AST. ZOOL, 
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 
a position that it closes the throat of the shell when the 
animal is inclosed, while the animal can push it on one 
side behind the plait formed by the groove above referred 
to, when it desires to protrude itself from its shell and 
walk about to search for food. The Siphonostoma only 
differ from the former in not having the plate-like tooth 
above described. Other genera, as Succinea , Amphibulina , 
and Bulimulus , have most of the characters of BuYimi , but 
differ in the edge of the peristoma being thin and acute. 
The group containing the Achatince, on the contrary, have 
the front of the mouth truncated, the axis of the shell 
imperforated in all its stages of growth, and the edge of the 
lips thin, whilst the tips of the upper tentacles of the ani¬ 
mal are acute and produced beyond the eyes. They gene¬ 
rally form a shelly epiphragma with a long impressed line 
near the outer hinder edge of the mouth of the shell, 
over the respiratory hole of the mantle; and they deposit 
very large eggs, which are generally covered with a hard 
shell. 
The family of Veronicellidce have the head retractile 
into a sheath formed by the front edge of the mantle; the 
foot, the sides of the body, and the four tentacles are con¬ 
tractile, and the lower pair of tentacles are bifid. The 
mantle is smooth, coriaceous, extending the whole length 
of the back and edging the foot. The vent and the open¬ 
ing to the pulmonary cavity, which is placed on the 
hinder part of the right side, is at the hinder extremity 
of the body, between the edge of the mantle and the 
foot. They live in forests under leaves. 
The family of Onchidiadce are very like the former, but 
the back is warty, and they have only two contractile 
rather club-shaped tentacles, and a broad lunate head; 
the respiratory organs are posterior, just under the edges 
