FORMATION OF SHELLS. 253 



successive layers on the internal surface, some- 

 times in an annular, and sometimes in a spiral 

 form. If an operculum were to be constructed 

 of a considerable size, and were connected to 

 the shell itself by a regular hinge, it would be 

 entitled to be considered as a distinct valve. 

 Here, therefore, we perceive, as was remarked 

 by Adanson, a connecting link between the 

 univalve and the bivalve testacea. A Clausium 

 is another kind of covering, serving also for pro- 

 tection, and consisting of a thin spiral plate of 

 shell, attached to the columella by an elastic 

 spring, by which the plate is retracted when the 

 animal retires into its shell. It thus corresponds 

 exactly in its office to a door, opening and closing 

 the entrance as occasion requires. An Epi- 

 phragma is a partition of a membranous or cal- 

 careous nature, constructed merely for temporary 

 use. It is employed for closing the aperture of 

 the shell during certain periods 

 only, such as the winter season, 

 or a long continued drought. 

 Fig. 118 exhibits the lines which 

 appear on the inner side of the 

 epiphragma, of the Helix pomatia, 

 or garden snail, and which in- 

 dicate the succession of deposits 

 by which it has been formed. 



It is remarkable in how short a time this 



