552 Wisconsm Academy of Sciences, Arts, and Letters. 
sunset to the third half hour after sunset. In Lake Mendota 
alone, the time of reaching the surface changed from the second 
half hour after 1 sunset to the second half hour before sunset in 
a single month, June, 1901. The time of leaving the surface 
in the morning varied from 1 more than two and a half hours be¬ 
fore sunrise to the first half hour after sunrise. Where there was 
a diurnal movement of the young, they generally came to the 
surface earlier in the evening and remained later in the morn¬ 
ing than the adults. 
Dapjtnia retrocurva Forbes. 
This species was found in four lakes and there was diurnal 
movement in two, Rainbow and Green lakes. In the former, 
there was a movement of two meters in July and six meters in 
September. In the latter, the young showed a, movement of 
six and adults ten meters. 
In Rainbow lake, adults came to the surface a little more 
than an hour after sunset and were not present in the surface 
catch made two hours; before, sunrise. In Green lake, adults 
came to the surface during the second half hour after sunset and 
were not found in the surface catch made an hour and a half 
before sunrise. 
Daphnia pulex De G. var. pulicarim Forbes. 
There was a movement of this form in two of the three lakes 
in which it occurred. In Oconomowoc lake, the movement of 
the adults varied in extent from twelve meters in October to one 
meter in June and in Okauchee lake, for the same months, 
from, ten to three meters. The young showed a movement 
ranging in extent from one meter to six meters. This variation 
in the extent of the movement was undoubtedly due to the 
change in the temperature of the water. D. pulicaria pre¬ 
ferred to remain in cool water in the daytime so 1 , when the 
upper stratum of water became too warm, it moved down to a 
greater depth. Thus the deserted zone near the surface gradu¬ 
ally grew in thickness as the water became warmer. After the 
formation of the thermocline, it occupied the lower part of this 
stratum and the region below and moved downward as the 
thermocline worked downward. For instance, the thermocline 
