Limnoplarikton 



441 



species of Ccelastrum are common and Ccelastrum reticulatum is mostly 

 confined to the plankton. Species of Tetradesmus and Ankistrodesmus 

 occur in some lakes, but never in quantity. Ankistrodesmus Pfitzeri is 

 a characteristic plankton-form in European lakes. Closteriopsis longissima 

 is frequent in many lakes, and Kircltrteriella lanaris and K. ubesa are often 

 notable constituents of the phytoplankton. Nearly all these members of the 



Fig. 271. Chart of the periodicity of five of the most conspicuous constituents of the phyto- 

 planktou of Wastwater, in the English Lake District. The temperatures are in degrees 

 Centigrade. 



Protococcales occur in greatest amount in the slightly contaminated lakes 

 and pools. From Victoria Nyanza Woloszynska ('14) has described a number 

 of other plankton-genera, such as Schmidleia and Xclrroederiella, but they are 

 only scarce plankton-constituents. Elakatoihrix gelatinosa is a rare Alga 

 which apparently occurs in very pure water, being known from the lakes of 

 Norway and from Wastwater (W. & G. S. W., '09 A). Two other species 

 occur in ponds and lakes in N. America, Europe and Asia. 



