440 Birge—Vertical Distribution of Pelagic Crustacea. 
day. Of the 59 sets of observations, 33 belong to the day and 
26 to the night. The number is about equal in each of the 
periods, except Period II, where the numbers are 15 for the day 
and 9 for the night. In any considerable series, this discrep¬ 
ancy in numbers would, of course, be present, since five of the 
eight daily observations are credited to the day. 
Six kinds of Crustacea were regularly present in the dredge: 
Diaptomus oregonensis Lillj.; Cyclops Uuckarti Sars; Cyclops 
brevispinosus Herrick; Cyclops pulchellus Koch; Daphnia hyalina 
Leydig; and Daphnia pulex, var. pulicaria, Forbes. These were 
present in very unequal numbers. On the average during the 
month the dredge caught 6556 Crustacea in each series of hauls from 
the six different levels. Of these, 64 per cent, or 4221 were Diapto¬ 
mus; a little over 2000 or 30.5 per cent, were Cyclops , including 
all species; 222 or 3.3 per cent, were D. hyalina , and 103 or 
1.5 per cent, were D. pulicaria. The tables which accompany 
this paper show the average number and the variation of 
each of these Crustacea in the different periods. It may, in 
general, be said here that Diaptomus was always by far the 
most numerous, ranging from 53 per cent, to 79 per cent, of 
the total catch, but declining somewhat both in absolute num¬ 
bers and in percentage in the later series of observations. 
Cyclops ranged in the different periods from 13 to 40.5 percent, 
of the catch, increasing in percentage in each period to the 
end. This increase in numbers of Cyclops continued later in 
the year, until the latter part of October or the first of Novem¬ 
ber, while Diaptomus fell off rapidly in numbers during August. 
D. hyalina constitutes from 2.3 per cent, to 6.4 per cent, of 
the total catch. Its numbers were about stationary during the 
month, but showed later a rapid and very great increase. It 
became the leading crustacean, as the species reached in October 
a maximum catch of over 8000 individuals in a single series of 
hauls. D. pulicaria in the first three periods yielded between 
1.5 and 2 per cent, of the entire catch, with an average of 125 
individuals, while in the last period it gave only about .3 per 
cent., averaging 18 individuals. Later this species practically 
disappeared entirely from the lake, so that it is probable that 
we are here dealing with the last of its development. 
