HELIX. 
101 
These animals have a distinct and very variously 
divided vesicula mul- 
tifida^ which is want- 
ing in Succinea^ Bu-- 
limus^ and other allied 
genera. 
The young shells Helix cantm 
have the outer whorls 
generally more or less keeled^ and the axis is always 
umbilicated or perforated; but the perforation is 
sometimes masked by the reflexion of the outer lip 
of the adult shell over it. 
This genus is known from Zonites by the thick- 
ening of the outer lip^ from Vitrina by the axis 
being perforated, from Succinea and Bulimus by the 
axis being depressed (and not elongate), making the 
shell subglobose or depressed. 
The animals, at the approach of winter, or in very 
dry weather in summer, recede into their shell, and 
secrete a quantity of mucus, which being moulded, 
as it were, on the retracted part of the mantle which 
encloses the folded-up foot, forms, when it dries by 
exposure, a cover to the aperture, which is usually 
membranaceous, with a triangular perforation over 
the respiratory hole of the mantle. 
In some species, as Helix Pomatia, the membrane 
becomes strengthened with a quantity of calcareous 
matter, which is first deposited on the triangular spot 
before referred to. In this case, the animal forms 
several membranaceous coverings, a little distance 
from one another, within the outer, hard, calcareous 
one, similar to the membranaceous covering of other 
Fig. 34 . 
