HOW TO KNOW THE FRESH-WATER ALGAE 



362a Wall thick; plates easily seen, with a suture between the plates 



usually evident; transverse furrow completely encircling the cell. 



363 



362b Wall thin; plates seen with difficulty (especially in filled and liv- 

 ing cells); transverse furrow completely encircling the cell or not. 

 364 



363a Wall with 2 antapical plates (plates at the posterior pole, to be 

 seen in end view of the cell); cell slightly flattened dorsiventrally 

 in most species. Fig. 258 PERIDINIUM 



Figure 258 Vi 

 Figure 258 " 



Fig. 258. Peridinium wisconsinense Eddy, a, ventral view showing 

 longitudinal sulcus; b, dorsal view; c, posterior polar view showing 

 2 antapical plates. 



Fig. 258 Vi. Encysted Dinoflagellates. 



This genus is represented by more species in fresh water than any 

 of the other dinoflagellates. They are differentiated by shape, size and 

 details of plate number and arrangement. The 2 posterior or antapical 

 plates can be determined by patiently rolling the cell so that it can 

 be seen from the bottom. Although very common, these species do not 

 form blooms nor pure growths as does Ceratium (Fig. 256). 



363b Wall with 1 antapical plate; cell not flattened dorsiventrally (round 

 in cross section). Fig. 259 GONYAULAX 



Fig. 259. Gonyaulax palustre Lemm. 



These almost spherical cells are differentiated 

 from Peridinium (Fig. 258) by the presence of but a 

 single antapical plate and the slightly spiralled 

 transverse furrow. Some authorities regard this 

 particular species of the genus as belonging to 

 Peridinium. 



Figure 259 



152 



