14 



POPULAR CONCHOLOGY. 



from injury when the animals are alive, but it soon rubs 

 off after death. It seems to be composed of animal matter 

 alone. 



Many of the turbinated or spiral-shelled Mollusca have 

 the power of fomiing an additional protection to their 

 bodies, by constructing a 

 kind of door, called the oper- 

 culum* , or epiphragma^, 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 wea- 

 ther, at the end of that time 

 throw it off, and on a re- 

 currence of the necessity make another; these being, 

 therefore, of a temporary nature only, are slight and easily 

 formed, but there are other genera which have them per- 

 manently attached, and of a more solid description t : these 

 latter are fixed to the foot of the animal, and are en- 

 larged in size by the mollusc, as the shell is increased, 

 in order that they may correspond with and close the 

 aperture. They are found attached to the animal before 

 the egg is hatched. In the foregoing figure are delinea- 

 tions of several varieties of opercula. Some are shelly, 

 and others of a horny construction. 



The beautiful and varied colours which shells present 

 are produced by colouring matter contained in the pores, 



a. concentric. 



Opercula. 



b, spiral. c, claw-shaped. 

 d, articulated. 



* From operculum^ Lat., a cover or lid. 

 "|" From epiphragma, Gr., a stopper. 



| The temporary clausilia, or doors, are called epiphragmata, the 

 permanent, opercula. 



