FORMATION OF SHELLS. 173 



formed, in the course of twenty-four hours, a fine pellicle, 

 resembling a spider's web, which is extended across the va- 

 cant space, and constitutes the Hrst stratum of the new shell. 

 This web, in a few days, is found to have increased in thick- 

 ness, by the addition of other layers to its inner surface; and 

 this process goes on until, in about ten or twelve days, the 

 new portion of shell has acquired nearly the same thickness 

 as that which it has replaced. Its situation, however, is not 

 exactly the sam.e, for it is beneath the level of the adjacent 

 parts of the shell. The fractured edges of the latter remain 

 unaltered, and have evidently no share in the formation of 

 the new shell, of which the materials have been supplied 

 exclusively by the mantle. This Reaumur proved by in- 

 troducing through the aperture a piece of leather underneath 

 the broken edges, all round their circumference, so as to lie 

 between the old sliell and the mantle: the result was that no 

 shell was formed on the outside of the leather; while, on the 

 other hand, its inner side was lined with shell. 



The calcareous matter which exudes from the mantle in 

 this process is at first fluid and glutinous; but it soon hardens, 

 and consolidates into the dense substance of the shell. The 

 particles of carbonate of lime are either agglutinated toge- 

 ther by a liquid animal cement, which unites them into a 

 dense and hard substance, resembling porcelain; or they are 

 deposited in a bed of membranous texture, having already 

 the properties of a solid and elastic plate. This explains 

 the laminated structure possessed by many shells of this 

 class, such as that of the oyster, of which the layers are easi- 

 ly separable, being merely agglutinated together like the 

 component leaves of a sheet of pasteboard. 



It has long been the prevailing opinion among naturalists 

 that no portion of a shell which has been once deposited, and 

 has become consolidated, is capable of afterwards undergoing 

 any alteration by the powers of the animal that formed it. 

 Very conclusive evidence has, in my opinion, been adduced 

 against the truth of this theory, by ]\lr. Gray, in a paper 

 lately read to the Royal Society. From a variety of facts, it 

 appears certain that on some occasions the molluscous animal 



