13 



dd. Transverse ribs of the valves extending deep into the cell cavity. 



Dcnticula. 



cc. Cells prevailingly much shorter than broad (rod-shaped, the longer 



axis of rod representing transverse axis of cell) often united into 



filaments. 



d. Cells cuneate in girdle view (/. e., valves not parallel) rachis median, 



interzones present. 



e. Valves punctate or variously punctate-striate, without transverse 



ribs ; cells not stalked. Sceptroneis. 



ee. Valves finelv transverse-striate and with transverse ribs (fig. ii). 



Meridian. 

 dd. Cells rectangular in girdle view, or if cuneate the rachis not median, 

 interzones present or absent, 

 e. Rachis median ; without central eye. 

 f. Valves with transverse ribs ; without a central "eye" (figs. 12, 13, 

 J , \ ( = Odontidium ) Diatoma. 



ff. Valves without tran.sverse ribs, 

 g. Ends of valves unequally swollen ; cells arranged radially (fig. 37). 



Asterionella. 



gg. Ends of valves alike, 

 h. Cells in filaments or zig-zag chains ; valves flat, without polar 

 nodules (fig. 15). Fragilaria. 



hh. Cells single, or forming fan-like stalked clusters. (Figs. 21, 22). 



Synedra. 



ee. Rachis near one margin, 

 f. Ends of valves alike, 

 g. Pseudoraphe and central nodule evident. Ceratoneis. 



gg. Pseudoraphe and central nodule not evident, (figs. 6, 7) 



Eunotia. 

 ff. Ends of valves unlike ( = AciincUa) Tibie/la. 



bb. Rachis otherwise ; raphe (slit) present, 

 c. Rachis containing an elongate slit (raphe) through the cell wall, 

 d. Rachis commonly median, often more or less lateral, not keeled or 

 when keeled not punctate, interzones present or absent, 

 e. Valves parallel, 

 f. Rachis of valves not keeled, 

 g. Raphe almost straight. 

 h. Raphe with a simple border, 

 i. vSepta of interzones (when present) not fenestrate, 

 j. Cells straight in girdle view. Including the subgenera Stauro- 

 neis (fig. 16), Frustulia (fig. 18) and Pinnularia (fig. 34). 

 (Figs. 24, 25,26,27). Naviada. 



jj. Cells curved, 

 k. Both valves with a raphe. R/ioiconns. 



kk. Only one valve with a raphe. Achtiatithes (including 

 Achnanthidiiim figs. 39, 40). 

 ii. Septa of interzones fenestrated, 

 j. Both valves with a raphe. Mastogloia. 



jj. Only one valve with a raphe (fig. 17). Cocconeis. 



