252 THE MECHANICAL FUNCTIONS. 



face of the preceding whorls. Thus we often 

 find that the ridges, ribs, or processes which had 

 been deposited on the surface of the shells of 

 the Triton, Miirex, &c. are removed to make 

 way for the succeeding turn of the spire. In 

 other cases, however, no such power of destroying 

 portions of shell previously deposited seems to 

 exist ; and each successive whorl is moulded 

 upon the one which it covers. 



It may also be observed, that some mollusca 

 have the means of excavating the shells of other 

 animals on which they may choose to fix, for 

 the purpose of forming a convenient lodgement 

 for themselves. The Pileopsis (or fool's cap) has 

 this faculty in a remarkable degree ; and it is 

 also met with occasionally in Siphonarice and 

 Patella;. The common Patella, or limpet of our 

 own coasts, often, indeed, forms for itself, by 

 some unknown process, a deep cavity out of a 

 calcareous rock. 



When the animal which inhabits a spiral shell 

 retires within it, the only part of its body that is 

 exposed to injury is that which is situated at the 

 mouth of the shell. With a view to its protec- 

 tion, it constructs, in many instances, a separate 

 plate of shell, adapted to the aperture, and deno- 

 minated an Operculum. This piece is constructed 

 by a process similar to that by which the rest of 

 the shell is formed ; that is, by the deposition of 



