SUB-CLASS III 



DIATOM AC E^E 



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 Desmidiece (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 Diatomacece (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 Phceophycccc, they are other- 

 wise much more nearly related to orders placed 



