SUB-CLASS III 
DIATOMACEH 
THIS great group, consisting of, it is estimated, 
some 10,000 species, and universally distributed in 
the waters of the globe, has been treated sometimes 
as a Natural Order allied to the Desmidiew (confined 
to fresh-waters), and sometimes as a sub-class, with 
the view of marking its peculiar characters, within 
the range of which there exists astonishing vari- 
ability of form without losing any of the essential 
points of character that distinguishes the group. 
They are given here the value of a sub-class, in 
recognition of the weight of this argument, while 
the familiar name of Diatomacee (which implies a 
Natural Order) is retained for overwhelming reasons 
of convenience; and their description is framed for 
similar reasons on the plan adopted for Natural 
Orders, since a subdivision of them would lead to 
an extended systematic treatment quite beyond the 
scope of this book. While their colouring-matter 
resembles that of the Phwophycew, they are other- 
wise much more nearly related to orders placed 
