HOW TO KNOW THE FRESH-WATER ALGAE 



153a Apex of semicell furnished with prominent spines; facial protuber- 

 ance (if any) one large low swelling, the wall thickened here and 

 often pitted or punctate. Fig. 115 XANTHIDWM 



Fig. 115. Xanthidium cristotum var. un- 

 cinatum Hass. 



This genus too, like Arthrodesmus 

 (Fig. 114) has cells that are compressed 

 so that they are narrow when seen from 

 the side or top. There is usually a 

 facial swelling in the center of the semi- 

 cell and all angles bear stout spines or 

 short arms that are tipped with spines. 



Figure 115 Some forms have granules on the wall. 



There is less variation in the shape of the semicell in this genus than 

 in some of the other desmids. 



153b Apex without spines or with a short, tooth-like spine at either 

 side. See Fig. Ill EUASTRUM 



154a (146) Cells spherical, inclosed by a spindle-shaped envelop which 

 has longitudinal ridges. See Fig. 97 DESMATRACTUM 



154b Cells not inclosed in such an envelope 155 



155a Cells oval, ovoid, spherical or ellipsoid 156 



155b Cells angular, pyramidal, trapeziform, or polygonal 183 



77 



