DIATOME^E. 21 



These accumulations of fossilized diatoms are used in the manu- 

 facture of dynamite and in various manufactures. 



The Diatomaceae appear nearest to, and must be placed as a 

 group co-ordinate with the Dinoflagellata, as they doubtless may 

 be supposed to derive their origin from forms resembling Exuviella, 

 and to have lost the cilia. The resemblances to the Desmidiaceae 

 which are striking in many respects, can only be conceived as 

 analogies, and cannot be founded upon homologies, and it is 

 therefore impossible to regard them as proof of genetic relation- 

 ship. The family contains only one order. 



FIG. 17. Various Diatomeae. A Synedra radians. B EpitJiemia turgida, (from the two 

 different sides). C Cymbella cuspidata. D Cocconeis pediculus (on the right several situated 

 on a portion of a plant, on the left a single one more highly magnified). 



Order 1. Diatomaceae. This order may be divided into two 

 sub-orders, viz. 



Sub-Order 1. Placochromaticae. The chromatophores are 

 discoid, large, 1 or 2 in each cell ; the structure of the valves 

 is bilateral and always without reticulate markings. The follow- 

 ing groups belong to this sub-order : Gomphonemece, Cymbellecc, 

 Amphorece, Acknanthece, Cocconeidece, Naviculece, Amphipleurece, Pla- 

 giotropidecr, Amphitropidece, Nitzchiece, Surirayece, and Eunotiece. 



Sub-Order 2. Coccochromaticae. The chromatophores are 

 granular, small and many in each cell. The structure of the cells 

 is zygomorphic or centric, often with reticulate markings. The 

 following groups belong to this sub-order : Fragilariece, Meridiece, 

 Tabellariece, Licmopkorece, BiddulpMece, Anguliferce, JEupodiscece, 

 Coscinodiscece and Melosirece. 



