458 Birge—Vertical Distribution of Pelagic Crustacea. 
Crustacea remained the same the density in different portions of 
the water was quite different. 
The average number of Cyclops caught in the different peri¬ 
ods greatly increases in the later periods over the earlier ones. 
The catch was as follows: 
Average. 
Minimum. 
Maximum 
Period I. 
626 
176 
1006 
Period II.. 
1611 
836 
3087 
Period III.. 
3147 
1800 
5460 
Period IV. 
2531 
1336 
4683 
It is thus evident that the number of Cyclops was increasing 
during the month of July. The percentage of Cyclops in the 
total number of Crustacea caught during the month was 30.5. 
In Period I, Cyclops averaged 13 per cent, of the total Crustacea; 
in Period II, 26.2 per cent.; in Period III, 27.4, and in Period 
IV, 40.5. This conclusion is confirmed by observations made 
later in the year. Three catches made in the latter part of 
August gave an average of more than 3000, and a maximum 
of more than 6000 Cyclops was reached in the latter part of 
September and the first part of October. Observations made 
in the upper levels of the water on the 23d of July, for the pur¬ 
pose of testing the horizontal distribution of the Crustacea, 
showed a large increase of Cyclops above the observations of 
the 19th, indicating apparently the development of a new gen¬ 
eration of Cyclops at that time. The falling off in number of 
Period IV, as compared with Period III, is probably not sig¬ 
nificant of any real change in the average number of the species. 
Vertical distribution .—In general the distribution of Cyclops 
agreed closely with that of Diaptomus , as may be seen on the 
accompanying plates and tables. The percentage of Cyclops in 
the upper level is not so great as that of Diaptomus , and the 
number found in the lower levels is somewhat greater. Yet the 
distribution of the two species agrees quite closely, as PI. IX 
shows at once. The upper 9 m. of the lake contain on an 
average from 78 per cent, to 93 per cent, of the total number. 
