ON MCLLUSCA. 169 
those whose canal or scissure is within, and these are the 
most numerous, have been produced by digitations of the 
mantle; those in which the scissure is external, as the horn 
of the purpura monoceros, and the spines of the corslet of the 
venus dionea, appear, on the contrary, to have been made by 
the concavity of an appendage of the mantle, which projects 
externally. 
But these lobules, these desections of the mantle, have not 
existed, as it would appear, at all periods of the active life 
of the animal, and then the shell could not have been furnished 
with corresponding desections. ‘This may be seen very well 
in the pterocéres and neighbouring genera, whose shell, in 
youth, much resembles that of a cone. We must then con- 
clude that in these genera the right lateral lobe of the mantle 
is dilated and widened, and sometimes digitated in rather an 
irregular manner, with age, and it is then that the shell pre- 
sents the wing or the digitations which characterize it. We 
must also necessarily admit that this disposition of the mantle 
diminishes, by little and little, at the period of old age, since 
the digitations of the shell, at first evidently canaliculated, 
are filled and solidified completely, and the right lobe of the 
mantle presents no trace of division at the places correspond- 
ing to those digitations of the shell now become solid. 
In a tolerably great number of mollusca, it appears that 
during the period of growth their activity suffers no interrup- 
tion, which probably is to be attributed to the constant reunion 
of favourable circumstances, more especially in temperature 
and nourishment ; and then the growth of the shell, more or 
less slow, is however uniform, until it has attained the swmmum 
of its development. But there are also many others of them in 
which, from the intermission of favourable circumstances, the 
animal, being forced to diminish the intensity of its vital acti- 
vity at certain periods of the year, or of its life, the shell pre- 
sents indications of these periodical intermissions in the swel- 
