FORMATION OF SHELLS. 179 



row 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 recognised as belonging to the same tribe 

 of mollusca. This is remarkably the case with the shell of 

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

 growth, (Fig, 112) has the ordinary form of an oblong tur- 

 binated 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 cor- 

 responding processes of the mantle.* But in this instance 

 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 dcposite, as they proceed, a dense and high- 

 ly polished porcellaneous shell, beautifully variegated with 

 coloured spots, which correspond exactly with tlie coloured 

 1X4 parts of the mantle that deposites 



them. This new plate of shell com- 

 pletely envelops the original shell, 

 giving it a new covering, and dis- 

 suisins: its former character. A 

 transverse section, (Fig. 114,) at once 

 shows the real steps by which these 

 changes have taken placet 

 Changes equally remarkable are observed to occur in the 

 interior of the shell at different stages of its growth. On 



• Similar chang-es occur in the shells of the Ovula (spindles,) Erato (tear- 

 shells,) and Margmella^ (dates.) Gray, Phil. Trans, for 1833. 



f According to Brug-uiere, there is reason to believe that the animal of the 

 Cy/Jra^a after having completed its shell, in the manner above described, still 

 continuing- to grow, and being incommoded for want of space, quits its shell 

 altogether, and sets about forming a new one, better suited to its enlarged 

 dimensions. It is stated also that the same individual is even cai)able of 

 forming in succession several shells. Blalnvillc, however, considers it im- 

 possible that the living animal can ever quit its shell. Malacologle, p. 94. 



