122 



Badllariese 



protuberance or a horn ; cells in cross-section mostly elliptic, more rarely 

 polygonal or circular. Includes the Chtetocerese, Biddulphiete, Anaulese, and 

 Euodiese. 



d. Rutilarioidese. Cells boat-shaped, with an irregular or radiating structure. 

 Includes the Rutilarieae. 



B. Pennatse. Valves zygomorphic, or less frequently irregular, never centric ; 



mostly boat-shaped or needle-shaped, with a structure arranged in relation to a raphe or 



pseudoraphe. 



Fragilarioidese. Cells mostly straight, rod-shaped or lanceolate, without a raphe, 

 but sometimes with a pseudoraphe or with indications of the commencement of 

 a raphe. Includes the Tabellarieee, Meridionepe, and Fragilariese. 

 Achnanthoidea. Cells crooked or suddenly bent, with a raphe on one valve and a 

 pseudoraphe on the other. Includes the Achnanthese and the Cocconeideee. 



Naviculoidefe. Cells with a raphe on each valve, with or without a keel in the 

 line of the raphe (sagittal line). Includes the Naviculese and Nitzschieee. 



Surirelloide& . Cells with the raphe concealed in submarginal wings of each 

 valve. Includes only the Surirelleoe. 



e. 



f. 



h. 



Fig. 87. Two small pennate diatoms. A C, Navicula muticopsis Van Heurck. D, Navicula 

 c/lobieeps Greg. All x 1500. A is a girdle view, B D are valve views. 



The classification put forward by Forti ('12) is based primarily upon the 

 fact that some diatoms are capable of spontaneous movements, whereas others 

 are not. Forti accepts O. Mliller's interpretation of these spontaneous move- 

 ments as being due entirely to protoplasmic currents circulating in the raphe, 

 and therefore the primary division separates all those diatoms which possess 

 a raphe (whether perfectly or imperfectly developed) from those which do 

 not. One improvement upon Schlitt's arrangement is the more definite 

 separation of the naviculoid diatoms from the Nitzschieae. The scheme is 

 briefly as follows : 



I. Immobiles. Diatoms incapable of spontaneous movement. Conjugation unknown 

 (perhaps non-existent) or possibly by the fusion of microspores. 



A. Valves usually circular, more rarely elliptic or reniform. Includes Melosirete, 

 Coscinodiscese, Asterolamprete, Heliopeltese, and Eupodiscege. 



B. Valves usually elliptic or lanceolate, sometimes linear, often regularly polymerous 



