ANNUAL DISTRIBUTION OF THE ORGANISMS. 3} 



Winnebago (Plate XIV) it appears as early as the middle of 

 May and reaches its maximum in July or August, then rapidly 

 diminishes, to disappear entirely the last of October. 



In Green lake it does not appear ordinarily until into July, 

 and reaches its maximum in August or September. Here, too, 

 it disappears the last of October. These results compare very 

 closely with those reached by Birge for Lake Mendota and 

 those noted by the European authors, Apetein, Fric and Vavra, 

 Burkhardt, Steuer, and Seligo. 



It will be noted that the season for Diaphanosoma in Green 

 lake is later than in Lake Winnebago, as we would expect, be- 

 cause of the more rapid warming of the shallower body of 

 water. The total number in Lake Winnebago, too, is greater, 

 and the maximum much more sharply marked. 



Daphnia liyalina Leydig and D. retrocurva Forbes. 



Under this head I have placed all the common limnetic Daph- 

 nias, without attempting a closer specific distinction. In Green 

 lake I think the form is retrocurva, while in Lake Winnebago 

 both hyalina and retrocurva are found. 



In Green lake Daphnm may be found at all times in the year 

 but between January and the last of May only in occasional 

 specimens. Its principal time of life is between the first of June 

 and the last of December. The results of the three summers 

 given in Plate XY, compared with the curves of my former 

 paper (Marsh '97), do not give sufficiently uniform results to in- 

 dicate maxima occurring' with any great regularity. In the sum- 

 mer of 1899 there were two distinct maxima, one the middle of 

 July and one about the middle of October. In 1900 the two 

 maxima were the last of August and the first of November, 

 while in 1901 there were three maxima, all less pronounced 

 than in the preceding years, one in July, one the latter part of 

 August, and one the first of November. The curves would 

 seem to indicate the common occurrence of two pronounced 

 maxima, one in July or August, and one in October or Novem- 



