HOW TO KNOW THE FRESH-WATER ALGAE 



356a Cells inversely triangular, or tetrahedral in top view, angles tipped 

 with 1 or 2 spines. Fig. 251 TETRADIN1UM 



Fig. 251. Tetradinium simplex Presc. a, side view; 

 b, vertical view. 



The chromatophores of this sessile member of 

 the dinoflagellates are typically brown. The four 

 corners of the cell are tipped with 2 short spines. 

 This and 1 other species, T. javanicum Klebs, have 

 been reported as epiphytes on filamentous algae. It 

 should be compared with flaciborsJcia (Fig. 252) in 

 making determinations. 



Figure 251 



356b Cells inversely triangular, elliptic in top view, the outer free angles 

 tipped with 1 stout spine. Fig. 252 RACIBORSKIA 



Fig. 252. Raciborskia bicornis Wo- 

 losz. a, side view, showing 

 stipe; b, top and lateral view. 



These are elliptical cells, sessile 

 on a short stalk and attached to 

 filamentous algae or aquatic moss- 

 es. There is a single spine at each 

 pole of the cell. One species only 

 has been reported from the United 

 States. Compare with Tetradinium 

 (Fig. 252) in making determina- 

 tions. 



Figure 252 



357a (355) Cells crescent-shaped, the horns extended to form a spine- 

 like tip which is recurved in most species. Fig. 253 



CYSTODINIUM 



Fig. 253. Cystodinium corniiax (Schiller) 

 Klebs. 



This and 2 or 3 other species occur 

 as free-floating members of the encys- 

 ted type of dinoflagellates. They are 

 differentiated on the basis of variations 

 of the crescent-shaped cells, with the 

 horns twisted at various angles. Sev- 

 eral species of Tetiaedron (Fig. 131) have been incorrectly described 

 from members of this genus. 



Figure 253 



149 



