FORMATION OF SHELLS. 213 



only in one point, there is formed, in the course of 

 twenty-fonr hours, a fine pellicle, resembling a spi- 

 der's web, which is extended across the vacant space 

 and constitutes the tirst stratum of the new shell. 

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

 in thickness, 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 same, for it is beneath the level of 

 the adjacent parts of the shell. The fractured 

 edges of the latter remain unaltered, and have evi- 

 dently no share in the formation of the new shell, 

 of which the materials have been supplied ex- 

 clusively by the mantle. This Reaumur proved by 

 introducing through the aperture a piece of leather 

 underneath the broken edges, all round their cir- 

 cumference, so as to lie between the old shell 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 together 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 easily separable, being 



