248 THE MECHANICAL FUNCTIONS. 



a dense and highly polished porcellaneous shell, 

 beautifully variegated with coloured spots, which 

 correspond exactly with the coloured parts of 

 the mantle that deposits them. This new plate 

 of shell completely envelopes 

 the original shell, giving it a 

 new covering, and disguising 

 its former character. A trans- 

 verse section (Fig. 114) at 

 once shows the real steps by 

 which these changes have 

 taken place.* 

 Changes equally remarkable are observed to 

 occur in the interior of the shell at different 

 stages of its growth. On the inner surface of 

 the Mitra, the Volute, and other shells of a similar 

 kind, there is deposited a layer of a hard semi- 

 transparent calcareous material, having a vitreous 

 appearance.! The thickness of the layer, which 

 thus lines the cavity of the shell, is greater as it 

 approaches the apex ; and where the spire is 



* According to Bruguiere, there is reason to believe that the 

 animal of the Cypr&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 

 capable of forming in succession several shells. Blainville, 

 however, considers it impossible that the living animal can ever 

 quit its shell. Malacologie, p. 94. 



f This is the substance represented at D, Fig. 107, p. 234. 



