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 the'^most 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. 
1895. 
1896. 
June 16-30.. 
s 
a 
July 1-15... 
9.7 
s 
July 16-31. 
31.5 
2.5 
August 1-15 — .. 
August 16-31. 
68.2 
50.1 
27.6 
57.1 
September 1-15..... 
23.8 
157.7 
Spsptfimher 16-30. 
53.6 
228.6 
October 1-15... 
72.5 
299.3 
October 16-31... 
70 g 
59.3 
92.7 
November 1-15... 
9.9 
November 16-30... 
24.2 
s 
December 1-15. 
5.0 
s 
December 16-31.... 
0.7 
a 
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 
