lO 



e. Cells in vertical view radiating, triangular, quadrangular, or radiate ; 

 rarely fusiform (fig. 13, 14). Staurastrjiiti. 



ee. Cells compressed (at right angles to the plane of the front view), in 

 the vertical view fusiform or elliptical, 

 f. Cells lobed or incised, 

 g. Cells mostly oblong or elliptical ; moderately lobed ; margins 

 wavy, the depressions rounded (figs. 24, 26). Euastrum. 



gg. Cells very much compressed, mostly orbicular or broadly ellipti- 

 cal, deeply lobed or incised (fig. 15, 18). Micrasierias. 

 ff. Cells with a more or less entire margin, often furnished with warts 



or spines. 



g. Cells without spines (figs. 21, 22). Cosmarimii. 



gg. Cell wall with regularly arranged spines of considerable length. 



h. Spines several, commonly in pairs ; a central protuberance 



always present. Xanthidiiim. 



hh. Spines 4 or 8, occasionally 16, no central protuberance. 



Arthrodesiniis. 

 bb. After division the cells remain attached to form colonies, 

 c. Colonies spheroidal ; cells not in contact, but joined by gelatinous 

 bands, 

 d. Gelatinous bands narrow ; few cells forming a microscopic colony. 



Cosmocladiii in. 

 dd. Gelatinous bands very broad, many cells forming a microscopic 

 colony. Oocardiunt. 



cc. Colonies threadlike ; cells attached by their apices to form long fila- 

 ments. 

 d. The line of division of the cell where the new and old parts of the 

 cell wall are fitted together, does not develope a girdle during 

 division, 

 e. Cells attached by special apical processes, 

 f. Apical processes very short. Sphccrozosoma. 



> ff. Apical processes long and overlapping the apices of the adjoining 



cells. Oiiychonenia. 



ee. Apices of cells plane and flat, 

 f. Cells slightly constricted. Hyalotheca. 



ff. Cells deeply constricted ; in vertical view elliptical. Spondylosiuiii. 

 dd. The line of division of the cell, developes a girdle during division, 

 e. Cells short, in vertical view fusiform, triangular, or quadrangular, 

 rarely circular with produced angles (fig. 9). Desmidiiim. 



ee. Cells elongate, cylindrical. Gynmozygas. 



Bacillariaceae {Diatorns.y^ 



a. Cells in transection circular, less commonly polygonal or elliptical, and 

 rarel}' irregular ; valves marked concentrically or radiately by dots, 

 areolations, lines or ribs ; cells often with spines, processes or horns. 



CENTRIC-K. 



Adapted from Bessey. 



