FORMATION OF SHELLS. 249 



much elongated, or turrit ed, as it is called,* this 

 deposition entirely fills the upper part, which, in 

 the early condition of the shell, was a hollow 

 space with thin sides. The purpose answered 

 by this deposit is evidently to give solidity and 

 strength to a part which, by remaining in its 

 original state, would have been extremely liable 

 to be broken off by the action of the sea. 



In other cases a different expedient is adopted. 

 The animal, instead of fortifying the interior of 

 the apex by a lining of hard shell, suddenly 

 withdraws its body from that part, and builds a 

 new wall or partition across the cavity, so as to 

 protect the surface thus withdrawn. That por- 

 tion of the shell, which is thus abandoned, being 

 very thin and brittle, and having no support in- 

 ternally, soon breaks off, leaving what is termed 

 a decollated shell ; examples of this occur in the 

 Cerithium decollation, the Bulimus decollatus, 

 &c. The young of the genus Magilus has a 

 very thin shell of a crystalline texture ; but 

 when it has attained its full size, and has formed 

 for itself a lodgment in a coral, it fills up the 

 cavity of the shell with a glassy deposit, leaving 

 only a small conical space for its body ; and it 

 continues to accumulate layers of this material, 

 so as to maintain its body at a level with the top 

 of the coral to which it is attached, until the 



* As in the genera Turritella, Terebra, Cerithium, and Fas- 

 ciolaria. 



