FORMATION OF SHELLS. 239 



bling porcelain ; or they are deposited in a bed 

 of membranous texture, having already the pro- 

 perties 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 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 consoli- 

 dated, 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 Mr. Gray, in a paper lately read to 

 the Royal Society. From a variety of facts, it 

 appears certain that on some occasions the mol- 

 luscous animal effects the removal of large por- 

 tions of its shell, when they interfere with its 

 own growth, or are otherwise productive of in- 

 convenience. We should at the same time re- 

 gard these cases in the light of exceptions to the 

 ordinary rule that a portion of shell once formed 

 remains ever after unchanged, while it continues 

 to be connected with the animal which produced 

 it. In a general way, indeed, we may consider 

 the connexion between the animal and tlie shell 

 as mechanical, rather than vital ; and the shell 

 itself as an extraneous inorganic body, forming 



