ANNUAL DISTRIBUTION OF THE ORGANISMS. 27 



Birge reports for Lake Mendota that the largest collections 

 were made in May. This does not correspond to the results of 

 either Green lake or Lake Winnebago, although in one year there 

 was a considerable rise in Lake Winnebago in May. In gen- 

 eral it may be stated that C. brevispinosus is a perennial form, 

 flourishing especially in the months from May to October. 



Cyclops pulchellus Koch. 



Cyclops pulchellus does not occur in Green lake. 



In Lake Winnebago it is found from early October to the 

 last of June. As the curves show (Plate IX) it has no very 

 pronounced maximum, but it is evident that it is present in the 

 greatest numbers in May or June. This species is then, in 

 Lake Winnebago, distinctly a winter species. 



Distribution" of C. brevispinosus and C. pulchellus in Wis- 

 consin Lakes. 



While I have found C. pulchellus in Lake Winnebago only 

 in the months from October to June, inclusive, it is the com- 

 mon limnetic form in some other lakes. This is notably so in 

 the Great lakes. In certain of the Wisconsin lakes, so far as 

 my observations have gone, C. pulchellus is always present as 

 the limnetic form as C. brevispinosus is in Lake Winnebago 

 and Green lake. 



I have gone over very carefully the list of lakes, from which 

 T have had collections, to determine, if possible, what is the 

 factor which makes one lake a brevispinosus lake and another 

 a pulchellus lake, but so far without entire success. 



The following lakes, so far as I know, always have C. pul- 

 chellus: Lake Geneva, Elkhart, Chain o' Lakes, Cedar lake 

 Washington county, Birch lake, Stone lake, Sand lake, Lake 

 Michiganime, and Long lake Fond du Lac county. All 

 father examined hikes have brevispinosus. All the lakes hav- 

 ing pulchellus, with the exception of Sand lake, are of the 

 deeper lakes, and Sand lake has a maximum depth of fif- 

 teen meters, thus being one of the deeper lakes of this type. 



