This deposition of mucous, and of mucous mixed with calcareous matter, goes on as the animal grows and feels the want of a larger shell for its protection. The shell is, in fact, moulded on the body of the animal itself, as the body grows; and, for this reason, any irregularity 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 functions 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 commonly 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 mucous 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 shell. 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."
- Land and Fresh-water Shells, 73.