206 BRITISH DESMIDIEJE. 



4. Cells granulate granulatum. 



Cells smooth 5 



5. Lobes of marginal cells terminated by obtuse processes .... ellipticum. 

 Lobes of marginal cells acute 6 



6. Marginal cells with notches as wide as the cells pertusum. 



Notches much narrower than the cells 7 



7. Marginal cells four simplex. 



Marginal cells more than four 8 



8. Six cells circularly disposed romid two central ones Napoleonis. 



Cells arranged in two or more circles 9 



9. Lobes of marginal cells angular, not tapering , angulosum. 



Lobes of marginal cells tapering Boryanum. 



20. SCENEDESMUS. 



1 . Cells obtuse 2 



Cells acute or acuminate 3 



2. Cells cylindrical or oblong quadricauda. 



Cells ovate obtusus. 



3. Cells ventricose at the middle 4 



Cells slender, not at all ventricose 5 



4. Cells in a single zigzag row acutus. 



Cells either in two rows or not zigzag obliquus. 



5. Outer cells crescent-shaped, inner straight, all approximate, dimorphus. 

 Cells curved, distant duplex. 



