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- 
