No. IO.] FRESH- WATER ALGM. 59 



front view), in the vertical view fusiform or el- 

 liptical ii 



Cells in vertical view radiating, triangular, quadran- 

 gular, or radiate; rarely fusiform Staurastrum* 



ii. Cells mostly oblong or elliptical; moderately lobed; 



margins wavy, the depressions rounded .... Euastrum* 



Cells very much compressed, mostly orbicular or 

 broadly elliptical, deeply lobed or incised . Micrasterias* 



Cells with a more or less entire margin, often 

 furnished with warts or spines 12 



12. Cell wall with regularly arranged spines of consider- 



able length 13 



Cells without spines Cosmarium* 



13. Spines several, commonly in pairs ; a central pro- 



tuberance always present .Xanthidium* 



Spines 4 or 8, occasionally 16, no central protuberance 



Arthrodesmus* 



14. Colonies spheroidal ; cells not in contact, but joined 



by gelatinous bands 15 



Colonies thread-like ; cells attached by their apices to 

 form long filaments 16 



15. Gelatinous bands narrow; few cells forming a micro- 



scopic colony Cosmocladium 



Gelatinous bands very broad, many cells forming a 

 macroscopic colony. Oocardiium 



16. The line of division of the cell, where the new and 



old parts of the cell wall are obliquely fitted to- 

 gether, develops a girdle during division 21 



The line of division does not develop a girdle during 

 division 17 



17. Cells attached by special apical processes 18 



Apices of cells plane and flat 19 



18. Apical processes very short Spharozosma* 



Apical processes long and overlapping the apices of 



the adjoining cells Onychonema* 



19. Cells deeply constricted 20 



Cells slightly constricted Hyalotheca* 



20. Cells in vertical view elliptical Spondylosium* 



Cells in vertical view quadrangular with produced 



angles Phymatodocis 



