30 PLANKTON OF WINNEBAGO AND GREEN LAKES. 



ber. The curves of my '97 paper indicate the same general 

 principle. 



In Lake Winnebago (Plate XVI) the maxima during the 

 summer months bear a fairly close resemblance to those of 

 Green lake, but there is, in addition, a pronounced maximum 

 in May or early June. This spring maximum may be due to 

 the increase of liyalina, as Birge' s results would indicate the ex- 

 istence of a spring maximum for this species. 



A comparison of the work of Birge on Lake Mendota and the 

 published results of the European authors Zacharias, Apstein, 

 Burckhardt, Seligo*, and Steuer shows that the occurrence of 

 limnetic Daphnia is very similar the world over, the winter 

 months having the minimum of production, and the greatest 

 numbers appearing in the summer and fall. The absence of 

 the spring maximum in Green lake is doubtless due to the slow 

 warming of the deep body of water. 



Daphnia pulex var. pulicaria Forbes. 



This species I have not found at all in Green lake. 



In Lake Winnebago it appeared May 11, 1900, and was 

 found in large numbers in June and then entirely disappeared. 

 In 1901 I found it in a single collection, — that of June 8. 

 When it occurred it was in the bottom waters, and was very 

 noticeable because of its size. It is possible that scattering in- 

 dividuals at other times were counted as hyalina, but if it oe- 

 curred at all it must have been in very small numbers. Birge 

 (Birge '97) has discussed in detail the occurrence of this 

 species in Lake Mendota, stating that it occurs there in num- 

 bers only in the odd numbered years. This does not seem to 

 be true in Lake Winnebago. He also states that it is confined 

 to the region of the thermocline, being limited above by the 

 high temperature of the water and below by the impurity of 

 the bottom waters. If this were true it would explain the dis- 

 appearance of this species in Lake Winnebago in summer, be- 

 cause this lake has no thermocline, and the whole body of water 

 is heated to a high degree. It would seem that the conditions 



