INTRODUCTORY CHAPTER. ^ 19 



Opercula. 



ing a kind of door, called the operculum*, or epi- 

 phragma], to the entrance of their shell. These 

 are made more or less strongly, as circumstances 

 require; some, as the snail and other land 

 species, merely needing this extra covering 

 during severe weather, at the end of that time 

 throw it off, and on a recurrence of the neces- 

 sity make another ; these being, therefore, of a 

 temporary nature only, are slight and easily 

 formed, but there are other genera which have 

 them permanently attached, and of a more solid 

 description \ : these latter are fixed to the foot 



* From operculum, a cover or lid. 

 f From epiphrasso, Gr., to stop up. 

 J The temporary clausilia, or doors, are called epi- 

 phragmata, the permanent, opercula. 



c 2 



