FORMATION OF SHELLS. 221 



rent part of the shell ; and at the same time narrow 

 its aperture, and materially alter its general shape 

 and aspect. Thus it happens that the shells of the 

 young and of the old individuals of the same 

 species are very different, and would not be recog- 

 nised as belonging to the same tribe of mollusca. 

 This is remarkably the case with the shell of the 

 Cyprcea, or Cowrie, which, in the early stage of its 

 growth (Fig. 112), has the ordinary form of an 

 oblong turbinated shell : but, from the process just 

 described taking place at a certain period, the 

 mouth of the shell (as shown in Fig. 113), becomes 

 exceedingly narrow, and the edges of the aperture 

 are marked by indentations, moulded on corres- 

 ponding processes of the mantle.* But in this in- 

 stance the change does not stop here; for both 

 edges of the mantle next take a wider expansion, 

 turning over the outer surface of the shell, and 

 passing on till they meet at the upper convex part, 

 or back of the shell, forming what has been termed 

 the dorsal line. They deposit, as they proceed, a 

 dense and highly polished porcellaneous shell, beau- 

 tifully variegated with coloured spots, which corre- 

 jj^ spond exactly with the coloured 



parts of the mantle that deposits 

 them. The additional plate, thus 

 deposited, completely envelopes 

 the original shell, giving it a 

 new covering, and disguising its 

 former character. A transverse 

 section (Fig. 1 14) at once shows 



* Similar changes occur in the shells of the Ovula (spindles), 

 Erato (tear-shells), and Marginella, (dates). Gray, Phil. Trans, 

 for 1833, p. 792. 



