472 Birge—Vertical Distribution of Pelagic Crustacea. 
any other catch from tins level. Almost all of the specimens 
obtained in this level were caught in the first two periods, 
where they averaged 34 and 25 respectively, while in Period III, 
an average of only two was obtained, and none at all in Period 
IV. The decline in numbers of Diaptomus and Cyclops in this 
level makes the absolute small number of D. pulicaria constitute 
an important percentage of the catch from this level. In 
Period I, 17.6 per cent, of the catch belonged to this species, 
and in Period II, 39 per cent. For the second period Cyclops 
with 34 specimens on the average was the only species which 
exceeded D. pulicaria in absolute number. In the third and 
fourth periods the number is so small as not to constitute any 
considerable fraction of the total catch. 
In the 15-18 m. level only 18 specimens were obtained 
throughout the entire month. Since all the catch from this 
level was counted, unquestionably all the specimens of D. p>uli- 
caria have been enumerated. It is highly improbable that the 
species visits the deeper part of the lake in considerable num¬ 
bers. No specimens in the control observations were obtained 
below 1 5 m. to a depth of 22 m. 
Diurnal distribution .—The number of D. pulicaria is so small 
that little need be said regarding its diurnal distribution. In 
general the species agrees with all the others which have been 
studied, in showing no constant variation between day and 
night. If we average the catches for the total period there is a 
slight tendency of the species toward the bottom by night, since 
in the day catches only 11.5 per cent, of the total catch was found 
in the 12-15 m. level, while nearly 22 per cent, of the entire 
catch was found in the same level at night. The absolute 
numbers, however, are so small and the quantity of the catch 
so dependent upon chance, that no conclusion should be drawn 
from these results. In general, we may say that almost the 
entire number of this species occupied the space between 6 and 
15 metres both by day and night. 
No reason can be assigned at present for this singular verti¬ 
cal distribution. It will be observed that the distribution of 
this species is in striking contrast to that of D. hyalina from 
which it did not differ greatly in absolute number. While 
