2'2(j THE MECHANICAL FUNCTIONS. 



■i 



exhibits the lines which appear on 

 the inner side of the operculum of 

 the Turbo, and v\ hich indicate the 

 succession of deposits by which it 

 has been formed. The appearance 

 of the outer side is shown in Fig. 

 1 19. 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 be- 

 tween the univalve and the bivalve testacea. A 

 Clausium is another kind of covering, serving also 

 for protection, 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 Epiphragma 

 is a partition of a membranous or calcareous nature, 

 which has no adhesion to any part of the animal 

 that formed it, and is constructed merely for tempo- 

 rary 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. 



It is remarkable in how short a time the Helix 

 pomatia will construct this covering, when circum- 

 stances occur to urge its completion. On the 

 approach of winter, the animal prepares itself for 

 passing that season in a state of torpidity ; first, 

 by choosing a safe retreat ; and next, by retiring 

 completely within its shell, and then barricading its 

 entrance by constructing an epiphragma. This 



