HOW TO KNOW THE FRESH-WATER ALGAE 



186b Body of the cell abruptly narrowed to form horn-like processes. 

 Fig. 131 TETRAEDRON 



Fig. 131. a, Tetraedron limneticum 

 Borge; b, T. asymmetricum Presc; c, 

 T. lobulation var. crassum Presc; d, 

 T. regulare var. granulaium showing 

 chloroplast; e, T. regulare var. granu- 

 latum Presc; f, T. regulare var. bi- 

 furcatum Wille. 



This genus contains a large number 

 of species which vary considerably in 

 their shape and in the number of arms 

 Figure 131 or processes. Whereas some are sim- 



ple and have rounded angles, others 

 are polyhedral in shape and have varying degrees of lobings at the 

 angles. They occur both in the euplankton and in the tychoplankton. 



187a (185) Cells with 1 spine at each angle 



188 



187b Cells with 2 or 3 spines at the angles. Fig. 131b. . .TETRAEDRON 



188a Spines slender and needle-like. See Fig. 129. . .POLYEDRIOPS1S 



188b Spines broader at the base and stout, decidedly tapering. Fig. 

 132 TREUBARIA 



Fig. 132. Treubaria crassispina G. M. Smith. 



This free-floating plant is similar to Pachy- 

 cladon (Fig. 128) but the processes are not 

 darkly colored and are not toothed at the tip. 



Figure 132 



87 



