25. Cells narrowly elongate, with one cliloroplast in the cell which ex- 

 tends throughout its length; with a row of from four to six pyrenoids; 



lateral margins nearly parallel ( cells often slightly curved ) Roya 



PL 2, Fig. 8 



25. Cells stout, fusiform, the lateral margins convex; cells with two 

 chloroplasts, one in each half of the cell, bearing longitudinal, radiat- 

 ing plates; nucleus located medianly between the two chloro- 

 plasts Netrium 



PI. 2, Fig. 13 



26. Cells inclosed, several together, within a colonial mucilage, inter- 

 joined by fine, almost indiscernible fibrils Cosmocladium 



PI. 4, Fig. 13 



26. Cells not inclosed in a colonial mucilage 27 



27. Cells flat, nearly circular in outline, star-shaped in front view, or 

 disc-like; median incision very deep; semicells deeply lobed or in- 

 cised, these lobes often with secondary lobes and lobules Micrasterias 



PI. 4, Fig. 1, 2, 4, 5, 8, 9 



27. Cells not flat and disc-like in 'front' view; without star-like, radiating 

 lobes and lobules 28 



28. Cells with a shallow and broad, or a deep and narrow notch in 



the apices Euastrum 



PI. 3, Fig. 4, 5, 6, 9 



28. Cells without a notch or depression in the apex of the semicell (if 

 slightly retuse at the apex, then without a prominent facial swelling 

 in the midregion of the semicell which may be seen easily in vertical 



or side view if not in face view) 29 



29. Apex of the cell extended into three or more arms or lobes, the 

 arms usually extended radiately so that the cell appears star-shaped 



or triangular when seen in vertical or end view Staurastrum 



PI. 3, Fig. 8, 11, 13, 14 



29. Apex of semicells not extended into lobes or arms, or if with arms 

 then not radiate in three or more planes, the arms in one plane 30 



30. With extended arms at the apical angles; arms in one plane Staurastrum 



PI. 3, Fig. 10 



30. Without extended arms at the apical angles 31 



31. Margin of cells without spines although often furnished with gran- 

 ules or conical warts or horn-like thickenings 32 



31. Margin of cells with definite spines at the angles, sometimes spines 

 divided ( bi- or trif urcate ) 33 



32. Face of semicell with one, two or three protuberances or swellings 



as seen in front view; the protuberances usually with granules Euastrum 



PI. 3, Fig. 9 



32. Face of semicell without prominent protuberances (although in some 

 species there may be a low swelling) Cosmarium 



PI. 3, Fig. 1, 2, 3 



33. Face of semicell smooth Arthrodesmus 



PL 4, Fig. 6, 7, 10, 11, 12 

 33. Face of semicell with a protuberance or with granular decorations; 



not smooth 34 



[938] 



