ANNUAL DISTRIBUTION OF THE ORGANISMS. 15 



Pediastrum. 



I have found Pediastrum in Lake Winnebago in every month, 

 of the year except March and April. It is most abundant in 

 the summer and fall months but seems to have no pronounced 

 maximum. 



Staurastrum. 



Staurastrum is found in Lake Winnebago from May to No- 

 vember but never in any considerable numbers. 



Eudorina. 

 Eudorina I have found in small numbers in June and July. 



Merismopedia. 



Mei-ismo'pcdia was found in small numbers through the sum- 

 mer and fall of 1899 in Lake Winnebago. In 1900 it was 

 found only in the fall months, continuing to the middle of Feb- 

 ruary. In 1901, too, it did not appear until fall except in a 

 single collection in June. 



Clathrocystis aeruginosa Henfr. 



I have never found Clathrocystis in the plankton of Green 

 Lake. In Lake Winnebago it occurs throughout the summer, 

 appearing as early as June, and sometimes remaining as late 

 as October. I have found it in great numbers in Lake Buttes 

 des Morts in August. It appears in the same month in Peli- 

 can lake and the Eagle River lakes. It does not appear to be a 

 constituent of the plankton in the deeper lakes. 



Anabaena. 



Anabaena is one of the most important constituents of the 

 summer plankton. It occurs in Green lake, sometimes in 

 noticeable amount, so that little ridges of it are thrown upon 

 the shore by the waves. It is in the shallow lakes, however, 

 that it flourishes in especial abundance. It appears in Lake 

 Winnebago as early as May, and may be found as late as Octo- 



