INTRODUCTORY CHAPTER. 



13 



Pteroceras lambis. 

 ft, immature. b, mature. 



the mantle of the animal forming the extension of shell 

 with its accompanying pro- 

 jecting tubes, as a completion 

 to the habitation: it marks 

 the termination of the ani- 

 mal's labours, the shell is now 

 become perfect, and, the mol- 

 lusc having arrived at its full 

 growth, the secretion of shell 

 is no longer required to be 

 called into action for the pur- 

 pose of increasing the size of 

 its habitation ; and from this 

 period the animal, if it con- 

 tinue to produce shell, only adds to the thickness and 

 polish of its mansion, without extending its dimensions. 

 The Cowrie also when young has a thin, sharp-lipped 

 shell: in the adult, the lip is curled in, and very much 

 thickened. Some land snails, too, have a thick lip only 

 when the shell is arrived at full growth, Others have the 

 power of dissolving the inner portions of the shell, when 

 the body requires more space. " It should be added, that 

 all shells whose spires are exposed (and, being thin in their 

 young state, would be liable to be broken off by the action 

 of the sea,) have that part strengthened by the internal de- 

 position of shelly matter. Some few shells, however, as the 

 Bulimus decollates, for instance, lose the upper part of their 

 spire ; for the animal, instead of lining the upper whorls, 

 withdraws its body from them, and forms a division: the 

 communication of the body and the apex of the shell being 

 cut off, the latter part decays in the manner of a dead shell, 

 and falls off in particles."* Most shells have an outer 

 coating called the epidermis, which serves to protect them 



* Gray, Philosophical Trans. 1833. 



