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. 



Whelk. 



BIVALVE. 



MULTIVALVE. 



Mussel. 



Barnacle. 



