Forms, Composition, and Structure of Shells. 541 
increases in thickness and circumference. This, we feel per- 
suaded, is not the case; and is disproved by the examination 
of every univalve, and not supported by the structure of any 
bivalve. The first-formed whorls of the former are not thicker, 
not even in fact so thick, as the ultimate ones are; nor do any 
bivalves, so far as we are aware, gradually cua from the 
beak to the margin. The layers are only partial formations, 
and do not extend beyond the points to which the animal can 
protrude or retract the collar or the margins of the cloak. 
Bivalve shells are formed in precisely the same way as uni- 
valves; but here the margins of the cloak or mantle which 
embraces the body of the animal on each side perform the 
functions of the collar. 
You will have observed that the inner surface of all shells 
is very smooth, and apparently denser and_harder than the 
outer. It appears that the Mollusca, to form this inner layer, 
excrete the lime in a nearly pure state, that is to say, mixed 
with much less animal matter, so that in concreting the par- 
ticles become very close and compact, and receive a polish 
from the repeated frictions of the soft parts. ‘This structure 
is very brittle, and hence is called vitreous. It is often used 
to strengthen and finish the apertures of univalves, as is very 
remarkable in the helmet shells (Cassis Lamarck). ‘The upper 
part of the spire of some turreted shells are filled with the 
same matter at a late period of the animal’s life, and, be- 
coming dry and brittle, it is readily broken off, giving rise to 
what are called decollated shells. The particles which divide 
multilocular shells into several chambers are likewise vitreous, 
and seem to be formed by a deposition from the posterior part 
of the inhabitant at stated periods in its growth, the periods 
being regulated by a law of its or iginal constitution. 
In a great number of Molldsca it would appear that the 
increase of the shell from birth to mature size is uninterrupt- 
edly progressive ; but there are, perhaps, an equal number, in 
which the animal, at certain and determined intervals, forms 
a transverse rib or varix, and seems to become for a season 
inoperative. These ribs vary much in their numbers, in their 
figure, and in the distances at which they are placed in dif- 
ferent shells, but in the same species are alike and uniform, so 
that it would perhaps be better at once to refer their form- 
ation to a law of their individual life imprinted on them by 
their Creator, than to seek for its explanation in causes 
which are only partially applicable, or of doubtful existence. 
We may suppose, indeed, with Blainville, that, during the 
season of love, the derivation of fluid and of energy to the 
generative system may diminish the secretion of fluid and of 
