MOL.LUSCA. 



Trans, vol. liv. p. 43), erroneously considers the different 

 sizes of the cypreae 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 (Lee. cFAnat. 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 



