4 SYSTEMATIC DISTRIBUTION. 



and blunt, compared with the size of the shell. 

 They are always destitute of colour, for the animal 

 does not deposit the colouring matter until after it 

 has been hatched ; and it is therefore generally easy 

 to distinguish in the young shell (and sometimes also 

 in the adult) that part of the top of the spire which 

 formed the shell of the animal when in the egg. 



The shell is formed by the hardening of the animal 

 matter, which is secreted by certain glands on the 

 surface of the body, by means of chalky matter, 

 also secreted by similar glands. It has been stated 

 that the unhatched animal, very shortly after it 

 is formed, begins to make its shell; and when it 

 is hatched, deposits on the edge of the mouth of the 

 little shell which covered its body in the egg, a small 

 quantity of the mucous secretion. This dries, and is 

 then lined with some mucous matter, intermixed with 

 calcareous particles; and when this hardens, it again 

 places on its edge another very thin layer of the mu- 

 cous secretion, and again lines it as before. The 

 mucous secretion first deposited forms the outer coat 

 of the shell, which is called the periostraca or epi- 

 dermis, some persons having regarded it as similar 

 to the epidermis of the human skin. It is of use 

 in protecting it from injury; while the mucous 

 matter mixed with lime, which is placed within it, 

 *brms the substance of the shell itself, and consti- 

 tutes, with the calcareous matter already existing, 

 the crystalline structure of the shell. This depo- 

 sition of mucus, and of mucus mixed with calcareous 

 matter, goes on as the animal grows and feels the 

 want of a larger shell for its protection : the shell is, 



