GROWTH OF SHELLS. 15 



that of the epidermis which produces this envelope. The follicles, 

 ordinarily lodged in the edges of the mantle, deposit upon its 

 surface a semi-corneous matter, mixed with a greater or less pro- 

 portion of carbonate of lime, which moulds itself upon the sub- 

 jacent parts and solidifies. The lamina or layer, thus formed, 

 thickens and grows by successive deposits of new matter. Its 

 superficies is not stony, but in form of a kind of epidermis. 

 Sometimes it preserves a horny consistence throughout its whole 

 thickness. In general, however, the proportion of carbonate of 

 lime that it contains rapidly increases and imparts to it a stony 

 hardness. The internal surface is often harder than ihe rest, 

 and possesses a particular structure which renders it vitreous or 

 shining and nacreous. Sometimes the shell always remains en- 

 closed in the thickness of the skin ; but, in general, it is external, 

 and extends beyond the mantle, so as to afford the animal perfect 

 protection. Those which are unprovided with shells, or only 

 have an internal shell, are called naked mollusks, and those in 

 which the shell is visible are included under the name of con- 

 chifera, or sometimes testa'cea. 



9. The manner in which the shell grows is easy to under- 

 stand. If we examine the shell of an oyster, for example, we 

 observe that it consists of a multitude of superposed lamina?, the 

 separation of which may be effected by the assistance of heat. 

 These laminre are formed successively by the mantle of the ani- 

 mal which they cover, and, consequently, the most external one 

 must be the oldest; and it is also the smallest, and each new 

 lamina that is added, exceeds the lamina situate above, so that 

 while the shell is augmenting in thickness, it is also increasing in 

 size. In general, the distinctness of the lamina is less marked, 

 and often the new matter is deposited in such a manner, that its 

 molecules exactly correspond to the molecules of the matter 

 already consolidated, and thus gives to the whole a fibrous 

 structure. 



10. Shells are adorned with most diversified and most agree- 

 ably arranged colours, which often vary with their age. They 

 are almost always entirely superficial, and seem to depend on a 

 sort of a dyeing process performed by the skin of the animal, 

 which is coloured in a manner corresponding with that of its en- 

 velope. The colouring matter seems to be deposited upon the 

 shell at the time of its formation : and its brightness is in pro- 

 portion to the newness of the latter. It is produced by the edgo 

 of the mantle. When a shell happens to be broken and the ani- 



9. How does the shell grow? 

 1 0. IIoW are the colours of shells produced? 



