Conchology* 7 



ing, they do possess what may be considered as 

 a kind of pattern or model of the habitation which 

 they are to build. This appears to be of a pale 

 horn-colour, and destitute of any markings ; but as 

 soon as the animal enters upon an independent 

 state of existence, it begins to assume its distinctive 

 shape and colour, gradually increasing with the 

 growth of its living tenant, and becoming more 

 and more decidedly marked, until it attains its full 

 perfection of testacean development. Thus the age 

 of some shell-fish can be at once determined by the 

 peculiar conformation and markings of the shell. 



The relative portions of animal and earthy, or 

 rather chalky matter, which compose these shells, 

 vary considerably in different kinds ; in those 

 called Crystalline or Porcellaneous, the animal 

 deposit is much less than in the granular or con- 

 cretionary shells, where it not only constitutes a 

 large part of the whole substance, but is more 

 dense, that is, thick, and also has the appearance 

 of being membranous, or organized matter. We 

 can perhaps best explain this by saying that whereas 

 the different chalky layers of the crystalline shell 

 seem merely glued together by the intervening 

 animal fluid ; those of the granular shell, as the 

 oyster, appear to be connected by interlacing mem- 



