Forms, Composition, and Structure of Shells. 343 
sary; and in passing it repeatedly over the hole, leaves a layer 
of calcareous matter each time, until the new piece has ac- 
quired the requisite thickness. I will not deny that other 
parts may occ asionally secrete shell. The oper culum, in the 
tribes furnished with it, is certainly secreted by the part to 
which it is attached ; and the skin investing the ‘body « appears 
to furnish the material required for the for mation of the 
septa of multilocular shells, and the tubes of the Tubicole. 
The shell is to be considered as a model of the body of its 
architect ; so that, when we observe two shells to differ re- 
markably in their figure, we may infer, with perfect safety, 
that the animals differed no less; and it is this agreement 
between the living snail and its dead envelope which gives to 
the study of the Tate an importance it could not aiconse 
possess. Even the various processes, foliations, and spines, 
which vary and ornament the outer surface of shells, are the 
result of similar variations in the collar. ‘* The form of the 
shell,” I translate the words of Blainville, ‘‘ and even the pre- 
dominance of the animal matter over the earthy, bear a rela- 
tion to the form of the skin or of the cloak, and to the age of 
the snail. Thus, the tubular, spinous, and lamellated elonga- 
tions, which occur so frequently on the outer surface, are the 
consequences merely of prolongations, lobes, and fringes of 
the cloak; and the notches and channels of the aperture e are 
also produced by the habitual though intermittent protrusion 
of some organ, as of the respiratory tube, or of the head, or 
of the oviduct, &c.” * 
When, how ever, we come to apply the above general doc- 
trine of the formation of shells to particular vases, we meet 
with some apparent exceptions. The cowries (Cypre‘a), for 
example, by an addition of matter to the edges of their lips, 
would soon have the aperture entirely closed, as you will per- 
ceive on examining any species of that genus. To get rid of 
this difficulty, Bruguiére and others have imagined that the 
animal threw off the shell when it had become too small for his 
necessities, and then formed another more capacious, and bet- 
ter fitted for his ease. This theory labours under insurmount- 
able difficulty, nor does it seem required by the circumstances 
of the case. The Cypre’e, in their immature state, have a 
very different form from that they have when full grown. 
When young, they are very thin and brittle, with an evident 
spire, and a wide aperture, the margins of which are not 
* Manuel de Malacologie, p. 94.— I may here, once for all, acknowledge 
the great assistance I have received trom this work, in the compilation of 
this Letter. 
