400 FORMATION OF SHELLS. 



The theory, as thus expounded by Dr. Carpenter seems 

 to be the true one, and it recommends itself the more that it 

 can be easily reconciled with the experiments of Reaumur. 

 Indeed, the theory of the French naturalist differs in little 

 or nothing from the one now proposed except in considering 

 the excreted lime as consolidating itself from a mechanical 

 cause, instead of being moulded in the cells of a secreting 

 membrane. Hence there is much in the old doctrine which 

 needs no change of expression to suit it to the newer physi- 

 ology ; and this in particular is the case with that part of it 

 which relates to the colouring of the shell. The colour is 

 situated always in the outer layers, partially dimmed in many 

 by the periostracum, and, therefore, it must be furnished 

 solely from miliary glands situated in the collar or edges of 

 the mantle. All the varieties of colours, and all their varied 

 and mixed patterns which render shells so attractive and 

 pleasing, are the result of the arrangement of these glands, 

 and on their secretion of the colouring matter being uninter- 

 rupted, or interrupted at regular intervals. 



In a great number of Mollusca it would appear that the 

 increase of the shell from birth to mature size is uninter- 

 ruptedly progressive ; but there are, perhaps, an equal num- 

 ber, in which the animal, at certain and determined intervals, 

 forms a transverse rib or varix, and seems to become for a 

 season inoperative. These ribs vary much in their numbers, 

 in their figure, and in the distances at which they are placed in 

 different shells, but in the same species are alike and uniform, 

 so that it would perhaps be better at once to refer their form- 

 ation to a law of their individual life imprinted on them by 

 their Creator, than to seek for its explanation in causes 

 which are only partially applicable, or of doubtful existence. 

 We may suppose, indeed, with De Montfort and Blainville, 

 that, during the season of love, the derivation of fluid and 

 of energy to the generative system may diminish the secre- 

 tion of fluid and of lime from the cloak, and that then the 

 growth of the shell goes on as usual, as is indicated by the 

 plain intermediate spaces. We may further suppose, that, 

 when the seminal turgescence has subsided, the fluids are 

 carried in greater abundance to the skin, whence an accumu- 

 lation of calcareous matter in the margins of the collar, and 

 a consequent varix or rib. We may make t such or similar 

 suppositions,* but they are idle and improbable ; and I men- 



* Lamarck supposes that on the addition of every new piece which the 

 growth of the animal obliges it to make to its shell, the Ranella comes out 

 and exposes itself for the entire length of an half volution, and thus remains 



m 



