SYSTEMATIC DISTRIBUTION. 5 



in fact, moulded on the body of the animal itself, as 

 the body grows ; and for this reason any inequality 

 in the body is moulded in the shell. 



The animal has the faculty, also, of mending any 

 break or injury that its shell may have received, if it 

 is not of such a magnitude as to derange all the func- 

 tions of the animal itself ; and it mends them in the 

 same manner as it forms its shell, that is to say, 

 by depositing first a coat of animal matter, and then 

 lining it with mucous matter, mixed with chalk to 

 harden it. But as the animal is usually very desirous 

 of getting the repairs done as quickly as possible, 

 and is most probably damaged by the injury it has 

 received, these repairs are generally much more 

 roughly executed than the shell itself, and com- 

 monly destitute of regular colour. 



The particles which vary the colour of the surface 

 of the shell are deposited, while the shell is being 

 increased in size, immediately under the outer mu- 

 cous coat ; and as these particles are also secreted by 

 peculiar glands, the colour is always situated in a 

 particular manner on each species, the glands being 

 gradually enlarged, and gradually separated, but 

 not changed in position, by the growth of the animal. 

 All the variations exhibited in the colouring of the 

 different species, or in the different individuals of the 

 same species, are produced by the permanent or 

 temporary interruption of the action of these glands. 

 But for a more detailed account of these phenomena, 

 I must refer the reader to my papers on the subject 

 in the Philosophical Transactions for 1833, reprinted 

 in Dr. Johnston's Letters on Conchology. 



