36 PLANKTON OF WINNEBAGO AND GREEN LAKES. 



I have made no systematic collections from other lakes, hut I 

 have found Chydorus in great numbers in Stone, Sand, and 

 Pelican lakes in September. 



I think we may say this, — that Chydorus is a perennial form, 

 reaching its greatest numbers in the months from July to No- 

 vember, with its maximum probably in October or November, 

 but with sometimes a very marked increase also in July. 



Leptodora hyalina Lilljeborg. 



Plate XX shows the annual distribution of Leptodora in 

 Lake Winnebago. It was entirely absent from both lakes from 

 November until May. In Lake Winnebago it occurs as early 

 as the beginning of May, but in Green lake it does not appear 

 until into July. It disappears in Green lake, too, early in • 

 September. In both lakes its principal occurrence is in the 

 months of July, August, and September. The occurrence in 

 Lake Winnebago corresponds veiy closely with Birge's state- 

 ment in regard to Lake Mendota, although he has found them 

 there as late as December. 



The observations of Burekhardt, Steuer, aud Seligo in Eu- 

 rope in regard to this species show that the occurrence there is 

 very nearly the same as in this country. 



Cypris. 



An undetermined species of ostracod was found in both 

 Green lake and Lake Winnebago collections. In Green lake it 

 was found in September and October of two years and in Feb- 

 ruary and March of one year. In Lake Winnebago its range 

 seems to have been from the first of April to the last of Septem- 

 ber. It was very much more abundant in the summer of 1899 

 than in that of 1900. It was not at any time present in suf- 

 ficient numbers to play any noticeable part in the total amount 

 of plankton. 



The Bloom. 



The phenomenon of the "bloom" or the "working of the 

 lakes," or the "breaking of the meres" as it is called in Great 



