466 Birge—Vertical Distribution of Pelagic Crustacea. 
Daphnia hyalina Leydig. 
This species was present in far smaller numbers than either 
of those which have been already described. The average num¬ 
ber taken was 222 for the whole series of observations, or about 
3.3 per cent, of all the Crustacea. The number varied from 42 
as a minimum to 541 as a maximum. In the different periods- 
the average varied quite irregularly. The average number was 
as follows: Period I, 312; Period II, 144; Period III, 274; 
Period IV, 260. It is probable that the large number included 
in Period I depended to some extent upon chance. The obser¬ 
vations made later in the year show conclusively that in July 
we have the beginning of the main development of D. hyalina. 
In August (3 observations) the numbers taken average 1000 or 
more, and in September and October a maximum of over 8000 
was reached. The numbers assigned to D. hyalina are probably 
not as accurate as those for Cyclops and Diaptomus , since the 
larger number of the former species was contained in the upper 
levels of the lake, where there were enormous numbers of 
Cyclops and Diaptomus. The number counted, therefore, de¬ 
pended somewhat upon chance. At the same time the numbers 
of D. hyalina do not vary more widely or more irregularly 
than do the numbers of D. pulicaria , of which species by far 
the larger number was found in the lower levels of the lake, 
where comparatively few Crustacea were present, and where a 
larger portion were counted than in the upper levels. 
Vertical distribution.- —Very little need be said of the vertical 
distribution of D. hyalina , since it agrees very closely with 
that of Cyclops and Diaptomus , more closely with the former. 
A glance at the curves of PI. VIII, where the percentile distri¬ 
bution of all the Crustacea is platted, will show Cyclops and 
D. hyalina agree in percentile distribution almost exactly. On 
the average about 42 per cent, was found in the 0-3 level, 30> 
per cent, in the 3-6 m. level, and 21 per cent, in the 6-9 m. level. 
Thus the upper 9 m. of the lake contain about 93 per cent, of 
the total catch. As a maximum there were found in these levels 
100 per cent., and as a minimum 47.16 per cent. 
The number of Daphnia hyalina fell off greatly below 9 m., 
the average catch in the 9-12 m. level being 13 (5.9 per cent.); 
