28 PLANKTON OF WINNEBAGO AND GREEN LAKES. 



On the other hand all of the lakes having brevispinosus, with 

 the exception of Green lake, are of the shallow lake type. In- 

 asmuch as pidchellus is found in the colder waters of the Great 

 lakes, it would be a natural inference to- suppose that its pres- 

 ence in Lake Winnebago in winter had immediate relation to 

 the winter temperature of the water, and that it might be found 

 during the summer in other lakes where the water retains a low 

 temperature through the year. It seems impossible, however, 

 to establish any such rule. Generally speaking it is true that 

 in the' shallower lakes brevispinosus is the common limnetic 

 form, while in the deeper lakes, which have a marked thermo- 

 cline in the summer we find C. pulchellus, but the exceptions 

 are so startling as to destroy most of the effect of the generaliza- 

 tion. 



Cyclops LeuckaHi Sars. 



C. LeuckaHi occurs in both Green lake and Lake Winnebago, 

 but at somewhat different periods. 



In Lake Winnebago it is perennial, but the numbers in win- 

 ter are very small. It is found in considerable numbers from 

 April to the last of October. It is apparent from the curves 

 of frequency (Plate X) that its maximum period is in July 

 and August, although in one year there was a great increase in 

 May. 



In Green lake, on the other hand, it appears to have a fall 

 maximum, although the total numbers observed were so 1 small 

 as to make one careful about drawing any exact inferences. 



The cause of the difference in the curves of the two lakes 

 is difficult to state. If the maximum of Green lake occurred 

 somewhat later than that in Lake Winnebago it might be 

 thought that the difference was due to the fact that the deeper 

 body of water is heated more slowly, but in the curve of 1900 

 the maximum in Green lake is late in the fall, so that this can 

 not be considered the real reason. 



