308 SUPPLEMENT 
At the end of a time, which varies more or less according 
to the species, and, perhaps, also according to circumstances, 
the eggs disclose, and a young snail issues forth, which is 
already invested with a shell, extremely thin, it is true, and 
almost membranaceous. Accordingly at this time they are very 
much afraid of the drying action of the air, and especially of 
that of the sun, and do not issue from the holes in which they 
have been born, except during the night. Their growth at 
first is tolerably quick, but afterwards it becomes much slower, 
so that judging by the striz of the shell, which mark the 
growth, these animals ought to live along time. But with 
regard to this point we possess no certain information. ‘The 
erowth of their body, in fact, necessitates a proportional 
erowth in the shell. When it is necessary that this shell 
should be increased, the helices assemble in troops. ‘The 
animal remains in a state of repose, sinks into some cavity, 
and there issues from all the parts of the mantle, and espe- 
cially from its thick edge, a stratum of calcareo-glutinous 
matter, which is applied within the preceding, out-edging 
it a little. It is this part of the junction of this new 
stratum which forms the stria that marks the growth; it is 
so much broader in proportion as the animal is_ better 
nourished and more vigorous. When the shell is arrived at 
the completest growth which it can attain, it merely becomes 
thicker, and forms, in most of the species, a sort of pad, more 
or less thick ; and there is deposited on that part of the spire 
which modifies the aperture, a calcareous matter, generally 
not very thick, which can join the two edges. This is named 
a callosity. ‘Though all this constitutes the complete or ter- 
minated shell, yet the animal was adult before, that is to say, 
was capable of reproducing its species. But it is very im- 
portant to observe, that the shell of the same individual 
differs much according to the period of life of the animal 
which is examined. In general the spire is so much the less 
