233 
page 228) exist also in the case of Certodaphma and in much the 
same form. 
The relation of temperature to Ceriodaphnia is evident in its 
seasonal distribution. It does not advance rapidly in its vernal 
increase until after the water warms to 70°, and drops suddenly in 
numbers when the autumnal decline passes this point. Moreover, 
seasonal variations in temperature are accompanied by correspond- 
ing shiftings of the pulses of Certodaphmia. Thus in 1898 the water 
did not reach 70° until about May 20, reaching 73° on May 24, and 
the vernal pulse of Certodaphnia began at once its rise to the maxi- 
mum of June 7. In 1896 spring was early, 72° being recorded in 
surface waters on April 24, and we find a vernal pulse rising to a 
maximum on May 8. So also in 1897, when high temperatures 
continued into the autumn, the decline passing 71° on October 5, 
instead of in the first half of September as in other years, we find 
the pulses of Ceriodaphnia extending into October with unusual 
amplitude, reaching 5,200 per m.* October 5, while the highest 
record in this month, or later, in other years was 280 perm.? Tem- 
perature rather than season is thus the dominant factor in the 
seasonal curve of occurrence of Certodaphma. 
The form of this seasonal curve is typically that of a series of 
recurrent pulses of varying magnitude tending to reach the maxi- 
mum height in the vernal pulse of May—June, attaining often lower 
levels in July and rising again in August-September, and falling to 
Pemuinimum, or even to disappearance, in October. These later 
pulses do not appear in the disturbed hydrographic conditions of 
1898 (Table I.), but are clearly delineated in the summer records 
of other years, especially in the stable conditions of 1897, where 
well-defined pulses appear in July, August, September, and October, 
at intervals of approximately four weeks, culminating July 14, 
August 10, September 14, and October 5. Their maxima attain 
respectively 5,600, 2,720, 6,000, and 5,200 per m.*, and the pulses are 
delimited in each case by minima of less than 500 per m.* They 
tend to coincide with those of other Entomostraca and to approach 
those of the Rottjera. 
The Certodaphnia population in channel waters is almost ex- 
clusively made up of parthenogenetic females. Males were not 
recorded at any time, though females with ephippial eggs appeared 
after the October pulse of 1897 and the vernal one of 1898. 
