8 MOLLUSCA. 
Trans. vol. liv. p. 43), erroneously considers the different 
sizes of the cyprez as depending on the thickness of the 
shell increasing according to age, without admitting a cor- 
responding increase of the dimensions of the contained ani- 
mal, or cavity for its reception. 
The shells of the first kind which we have noticed, from 
being formed of cones or layers applied to the inner edge of 
the margin, and extending beyond it, have an imbricated 
structure. Those of the second kind, consisting of layers 
regularly superimposed, have consequently a laminated 
. structure ; but between the two kinds there are numerous 
intermediate links, formed by a combination of the two pro- 
cesses. 
In some cases, the hard parts of the skin are not entitled 
to the appellation of shell, but may rather be considered as 
horn. Such are the coverings of the mandibles of the cut- 
tle fish, the branchial lid of the aplysia, and the operculum 
of the welk. The two last appendices, however, though 
horny in some species, are shelly in others. 
The position of the shell, with respect to the constituent 
layers of the integuments, exhibits very remarkable differ- 
ences. In some it appears instead of a cuticle, or at least 
without an external membrane investing it. In general, 
however, it occurs between the cuticle and the skin ; a po- 
sition which induced Cuvier (Lec. d Anat. Comp. xiv. 11.) 
to consider it as analogous to the mucous web of the verte- 
bral animals. Its intimate connection with the muscular 
system of the animal, and the protection which it affords, 
seem adverse to such a conclusion. In many species the 
testaceous substance occurs in folds of the corium, or insert- 
ed in its substance. In this position it never acquires the 
solid texture which shells exposed, or covered only by the 
