12 



b. Cells short box-shaped or discoid, mostly circular in transection, usually 



without horns or projections. 



c. Cells forming filaments, girdle side marked. 



d. Entire valve uniformly marked (figs, i, 2, 3, 4) { = Lysigonium). 



Melosira. 

 dd. Margin and center of valve differently marked ; central portion 

 areolated. Hyalodidya. 



cc. Cells single, girdle side not marked, 

 d. Without spines (figs. 9, 10). Cyclotella. 



dd. With circle of spines (figs. 5, 8). Stephanodiscus. 



bb. Cells of other forms. 

 c. Cells two to many times as long as broad, circular, rarely round ellip- 

 tical in transection ; girdle with numerous interzones. 



Rhizosolenia. 



cc. Cells box shaped, as long as broad or shorter, elliptical, sometimes 



lunate in transection ; valves with horns, valves with transverse 



septa, without spines. Terpsina\ 



la. Cells in transection narrowly elliptical to linear, less commonly broadly 



elliptical, lunate cuneate or irregular, valves marked pinnately or 



transversely by dots, areolations, lines or ribs ; cells without spines. 



Pennat.^. 

 b. Rachis of the valves (/. <?., the line between the divergent pinnate 

 markings) evident as a narrow unmarked strip (pseudoraphe), rarely 

 wanting; valve without slit (raphe, fig. v). 



c. Cells usually but little shorter than broad, or longer, with numerous 



interzones, mostly united into filaments. 



d. Transverse rib of the valves, when present, not extending into the 



cell cavity. 



e. Valves with a few prominent transverse ribs. Tetracylus. 

 ee. Valves finely transversely striate only ; pseudoraphe absent (figs. 



28, 36). {= Striate! la) Tabellaria. 



R 



CB, 



CB. 



V. — Transverse section of the frustule of 

 a Navicula. ( From Van Heurck, after 

 West), cb and cb^, the two connecting- 

 bands forming the girdle ; r, raphe ; en, 

 central nodule ; c, costa of valve. 



