18 



POPULAR CONCHOLOGY. 



the mollusc having arrived at its full growth, 

 the secretion of shell is no longer required to be 

 called into action for the purpose of increasing 

 the size of its habitation ; and from this period 

 the animal, if it continue to produce shell, only 

 adds to the thickness and polish of its mansion, 

 without extending its dimensions. " It should 

 be added, that all shells whose spires are ex- 

 posed, 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 deposition of shelly matter. Some few 

 shells, however, as the Bulimus decollatus, 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 di- 

 vision : 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."* 



Many of the turbinated or spiral univalve 

 Mollusca have the power of forming an ad- 

 ditional protection to their bodies, by construct- 



* From Mr. Gray, Philosophical Trans. 1833. 



