238 THE MECHANICAL FUNCTIONS. 



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

 of twenty-four hours, a fine pellicle, resembling a 

 spider's web, which is extended across the vacant 

 space, and constitutes the first 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 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 introducing through 

 the aperture a piece of leather underneath the 

 broken edges, all round their circumference, 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 gluti- 

 nous ; 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, resem- 



