FORMATION OF SHELLS. 225 



when the mouth of the spire is advancing over the 

 irregular surface of the preceding whorls. Thus we 

 often tind that the ridges, ribs, or processes which 

 had been deposited on the surface of the shells of the 

 Triton, Murex, &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 occa- 

 sionally in Siphonari^ and PatellcB. 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 ex- 

 posed to injury is that which is situated at the mouth 

 of the shell. With a view to its protection, it con- 

 structs, in many instances, a separate plate of shell, 

 adapted to the aperture, and denominated an Oper- 

 culum. 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 successive 

 layers on the internal surface, sometimes in an an- 

 nular, and sometimes in a spiral form.* Fig. 118 



* The curves exhibited in the operculum are logarithmic spirals, 

 like those of divided shells. See the paper of Mr, Moseley above 

 referred to in the note at page 218. 



VOL. I. Q 



