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 leuckarti 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. 



