324 Birge—The Crustacea of the Plankton. 
differ no more widely than catches might differ though made on 
the same day and close together. 
In each of the two years where the conditions of the preced¬ 
ing winter were known, the summer maximum was close to ten 
times the winter average. In all three years there was a 
marked decline of numbers to a late summer minimum in Au¬ 
gust; at which time the average number is to J of the max¬ 
imum. In 1895 there was a very marked drop in numbers 
about the first of July; while in 1896 the maximum number was 
maintained throughout June and early July and then there was 
a steady decline for a month or more. In 1894 observations began 
on the first of July. Diaptomus was practically stationary during 
the month and rapidly declined after the early part of August. 
These variations in number in different years are at present 
without complete.explanation. Yet the most singular fact — the 
notable drop in numbers about July first, 1895—certainly extended 
to the species all over the lake. Observations were made between 
the first and tenth of July in that year even in the remoter 
parts of the lake, and with substantially uniform results. What¬ 
ever the cause it was probably the same as produced a similar 
fall in the numbers of Daphnia hyalina at the same time. 
The autumnal condition of Diaptomus varies with the temper¬ 
ature of the early fall. In 1894 and 1896 there was substan¬ 
tially no recovery from the August minimum. 1896, indeed, 
showed minor variations of number but on the whole the num¬ 
ber did not increase. In 1895 on the other hand there was a 
very marked rise of numbers in September, culminating in the 
third week of that month. We shall hardly be wrong in at¬ 
tributing this additional brood of Diaptomus in 1895 to the 
higher temperature of the water in that year. There was very 
little decline of temperature until the very last days of the 
month as the following observations will show: 
1895. 
Sept. 2, 
6 a.m. 
Sept. 26, 
6 a. m. 
Sept. 30, 
6 a. m. 
0 meters... 
21.9° 
20.0° 
16.3® 
10 meters....... 
20.9 
20.0 
16.5 
18 meters. 
13.9 
17.7 
16.5 
