XI] 



OF CERTAIN OPERCULA 



521 



Turbo dwplicatus. 



Mean difference ^07207 



And -07207 is the logarithm of M805. 



Fig. 263. Operculum of Turbo. 



The logarithmic spiral is not only very beautifully manifested 

 in the molluscan shell, but also, in certain cases, in the little lid 

 or "operculum" by which the entrance to the tubular shell is 

 closed after the animal has withdrawn itself within. In the spiral 

 shell of Turbo, for instance, the operculum is a thick calcareous 

 structure, with a beautifully curved outline, which grows by 

 successive increments applied to one portion of its edge, and shews, 

 accordingly, a spiral line of growth upon its surface. The succes- 

 sive increments leave their traces on the surface of the operculum 



