PORCELLANEOUS SHELLS. 107 



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 sup- 

 plied exclusively by the mantle of the animal. This 

 Reaumur proved, by introducing through the opening 

 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, the inner side was lined with shell. The calca- 

 reous matter which exudes from the mantle in this pro- 

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

 into the substance of the shell. 



Some shells, called porcellaneous, have a more uni- 

 form and compact texture than those which are mem- 

 branaceous. The animal matter which unites the car- 

 bonate of lime is more equally blended with the earthy 

 particles, to which it appears to act as a cement, binding 

 them strongly together. In shells of this kind, the 

 carbonate of lime assumes more or less of a crystalline 



