Classification of Shells. 21 
two thousand five hundred have been described and 
classified ; the varieties, more or less distinct, are 
almost countless. Of shells found on and about the 
British Isles, there are about five hundred and 
fifty species, or, we should rather say were, for dili- 
gent inquirers into this branch of Natural History 
are almost daily adding to the number. 
We have already seen that shells are sometimes 
called Crystalline, and sometimes Granular, in 
accordance with certain peculiarities of construction 
before mentioned: this is one mode of division. 
There are several others made use of in different 
systems of arrangement, which only a deeper study 
of the subject than can be here entered upon would 
enable one to understand; the plainest and most 
common, however, is that which has reference to 
the form of the shell, which is one of these— 
UNIVALVE. BIVALVE, MULTIVALVE, 

Barnacle. 
