ISO THE MECHANICAL FUNCTIONS. 



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 much elongated, or turrited, 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 deposite is evident- 

 ly to give solidity and strength to a part which by remain- 

 ing 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 so as to protect the surface thus withdrawn.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 portion 

 of the shell which is thus abandoned, being very thin and 

 brittle, and having no support internally, soon breaks off, 

 leaving what is termed a decollated shell; examples of this 

 occur in the Cerithiurn, decollatum, the Bulimus decolla- 

 tus, &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 lodgement in a coral, 

 it fills up the cavity of the shell with a glassy deposite, 

 leaving only a small conical space for its body; and it con- 

 tinues to accumulate layers of this material, so as to main- 

 tain its body *at a level with the top of the coral to which it 

 is attached, until the original shell is quite buried in this vi- 

 treous substance. 



The forms of the Cone and Olive shells are such as 

 to allow but a small space for the convolutions of the 

 body of the animal, which accordingly becomes, in the 

 progress of its enlargement, excessively cramped. In or- 

 der to obtain more space, and at the same time lighten the 



* This is the substance represented at d, Fig. 107, p. 170. 



\ As iji the genera TurritdlUy Tercbra, Vcrithium, and Fa^ciolaria. 



