Daphnia retrocurva. 



345 



species is found in far greater abundance during the summer in 

 the cool, deeper water, and extends to the bottom of the lake. In 

 the lakes of the Oconomowoc group, this species is abundant 

 and is by far thejmost conspicuous of the Crustacea which are 

 found below the thermocline. 



Daphnia retrocurva. 



Figure 18. — Table H, Appendix. 



Table XX. — Number per sq. meter of surface stated in thousands. 



June 16-30 



July 1-15 , 



July 16-31 



August 1-15 



August 16-31 



September 1-15 . . 

 September 16-30. 



October 1-15 



October 16-31.... 

 November 1-15... 

 November 16-30 . . 

 December 1-15 . . . 

 December 16-31.. 



1895. 



s 



a 



9.7 



3 



31.5 



2.5 



68.2 



27.6 



50.1 



57.1 



23.8 



157.7 



53.6 



228.6 



72.5 



299.3 



70 8 



92.7 



59.3 



9.9 



24.2 



s 



5.0 



s 



0.7 



a 



1896. 



Daphnia retrocurva belongs to the periodic Crustacea, and its 

 numbers have been very different in the three seasons of my 

 study. In 1894 the species was practically absent; two 

 specimens only were seen in July, and none were found in later 

 months. In 1895 it was present in moderate numbers, and in 

 1896 the numbers in September and October were very consid- 

 erable. The small number in 1895 is probably the result of the 

 absence of the species in 1894. Perhaps also the competition of 

 Daphnia hyalina had something to do with preventing the in- 

 crease of the species in 1895. In that year Daphnia hyalina 

 was present in large numbers throughout the late summer and 

 the autumn. In 1896 D. hyalina declined greatly in numbers in 



