ANNUAL DISTRIBUTION OF THE ORGANISMS. 23 



ber, inclusive. D. sicilis is found from the last of September 

 or first of October until the first of July. D. sicilis is rarely 

 present in the summer months as D. minutus is seldom found 

 in the. winter and spring months. In considering the curve, 

 then, it must be remembered that the summer maxima are for 

 minutus while the winter maxima are for sicilis. In Novem- 

 ber and December and again the last of June the species are 

 found together in the plankton. In the three years under ob- 

 servation the greatest production of minutus was not far from 

 the first of August with a smaller fall maximum the last of 

 September or first of October. This differs from the only com- 

 plete curve in my former paper in that then the greatest maxi- 

 mum was in September. 



D. sicilis reaches its greatest numbers in February and 

 March. 



It may be remarked that this maximum of D. sicilis corre- 

 sponds with the maximum period of development of Asterio- 

 nelldj but whether there is any causal relation between the two 

 I am unable now to say. 



Diaptomus oregonensis Lilljeborg. 



Diaptomus oregonensis is the only form of the genus found 

 in Lake Winnebago, and is the common form in the greater 

 majority of Wisconsin lakes. The curves of annual occurrence 

 are shown in Plate III. It will be noticed that in the first 

 summer there were three marked maxima, the last of July, 

 the middle of September, and the first of November. There 

 was then a rapid decline, a constant winter minimum during 

 January, February, and nearly through the month of March, 

 when there commenced a slow increase in numbers. In the 

 second year the maxima were in the middle of August, the 

 early part of November, and a heavy maximum in the next 

 June. In the third year the summer maximum occurred the 

 first of September with a fall maximum the early part of No- 

 vember. In comparing the three years the uniformity of the 

 November maxima is very remarkable. With the July maxi- 

 mum of 1900 there is nothing to correspond in the other years. 



