15 PLANKTON OF WINNEBAGO AND GREEN LAKES. 



in the fall and winter, and reaches a maximum of numbers in 

 Miarch. 



In the European lakes, according to Apstein and Zacharias, 

 this genus occurs from April or March to August, but it would 

 appear from my collections that it may be found in our lakes 

 at any time except that in Lake Winnebago it fails at the hottest 

 time of the year. It would seem to be a fair presumption that 

 the heat of mid-summer is unfavorable to its growth although 

 it has been found in Green lake in great numbers in July. 



Synum uvella Ehrenberg. 



I have found Synura in Green lake only in two collections 

 made in May, 1901. 



In Lake Winnebago it sometimes forms an important ele- 

 ment in the winter plankton. I have found it in the months 

 f romi November to April, with the largest nuanbers in January, 

 February, and March. It seems to be very distinctly confined, 

 in its development, to the months when the lake is covered with 

 ice. 



Uroglaena. 



I have not found Uroglaena as a constituent of the plankton 

 of either Green lake or Lake Winnebago. I did find it, how- 

 ever, in large numbers in the plankton of the Eagle River lakes 

 in 'August, 1901 and 1902, and it seems probable that in some 

 shallow lakes it may be in the summer an important element 

 in the plankton. 



Cemiium Jiinmdinella 0. F. Miiller. 



Only once did I find Ceratium in Lake Winnebago in any 

 considerable numbers, in a collection made August 22, 1900. 

 It cannot be said ever to form an important part of the plank- 

 ton of this lake. It may be found at almost any time of the 

 year but always in small numbers. 



In Green lake it is distinctly a summer form. It is almost 

 entirely absent except in the months from July to October, in- 

 clusive. In the summer of 1899 its maximum was reached 



