HOW TO KNOW THE FRESH-WATER ALGAE 



150a Semicells with 2 extended arms at their apices as seen in front 

 view, narrowly elliptic when seen from the top. See Fig. 112 14. 

 STAURASTRUM 



Fig. 112V2. Staurastrum leptocladum Nordst. 



150b Semicells compressed or rounded when seen from the top or side, 

 not with radiating arms 151 



151a Margin of cell furnished with spines 152 



151b Margin of cell without spines, although sometimes granular. Fig. 

 113 COSMARIUM 



Fig. 113. a-c. Comarium panamense Presc; 

 b, side view; c, top view; d, C. margarita- 

 tum (Lund.) Roy & Biss., front and side view. 



Like the genus Staurastrum (Fig. 112) Cos- 

 marium includes several hundred species with 

 considerable variation in shape of semicell 

 and manner of wall decoration (granules, 

 teeth, scrobiculations). Nearly all are com- 

 pressed or rounded when seen from the side 

 or top, regardless of their shape when seen 

 from the front or broad side. 



Figure 1 1 3 



152a Face of semicell with protuberances or with the wall thickened 

 in the midregion (best seen when the cell is rolled to a lateral 

 view position) 153 



152b Face of semicell without swellings or protuberances. Fig. 114. 

 ARTHRODESMUS 



Fig. 114. Arthrodesmus incus (Breb.) Hass. 



This genus has compressed cells like 

 Cosmarium but the angles bear relatively 

 stout spines. The wall is smooth in this 

 genus, there being no granules, pits or 

 swellings. 



Figure 1 14 



76 



