ON MOLLUSCA. 295 
or in other words, to death. We are completely ignorant of 
the duration of its natural life, but it is probable that it is 
tolerably long, if at least we may judge by the duration of its 
growth, and because it passes its life under circumstances of 
no great variety. Nevertheless, we possess no positive data 
on this subject, and it must be confessed that it is exceed- 
ingly difficult to procure any. 
As to the duration of the shell, and the changes which it is 
susceptible of experiencing from the action of the air, and 
in the bosom of the earth, all this depends much upon its 
structure, its solidity, its bulk, and some accessory circum- 
stances. 
If it is exposed to the action of the air, and to the vicissi- 
tudes of temperature and humidity, it at first loses its colours, 
which are very quickly changed (the ferrugineous resist the 
longest), and it usually becomes of a tarnished white; the 
animal matter is destroyed, and disappears by little and little. 
The composing lamine being no longer connected, exfoliate, 
expecially from the alternation of heat and cold; and soon, by 
this continual action, the lamine themselves become resolved 
into a sort of calcareous dust, which is drawn along by the 
currents of the water. 
The particular structure of the shell, its age, and even its 
bulk and thickness, facilitate or arrest more or less its terreous 
decomposition. 
If, on the contrary, the dead shells are, by particular cir- 
cumstances, sunk in the sand, or in the mud, in which they 
have lived, and where they have been encrusted with a creta- 
ceous deposition, which takes place in greater or less quanti- 
ties in all fresh and salt-water, but especially in the first; or, 
finally, if by the action of currents they are accumulated 
together, whether broken or not, in some localities of seas or 
lakes, as in these different cases they are placed in shelter 
from the vicissitudes of temperature and humidity, their de- 
VOL, XII. Q 
