442 Birge — Vertical Distribution of Pelagic Crustacea. 
no observations as yet show the time when this very distinct 
tendency of Crustacea toward the upper part of the water begins 
or the date at which it has become established. 
In observations made on lake Plon, and printed in the 3d Report 
of the Biological Station (p. 137), Zacharias gives a few obser¬ 
vations regarding the number of Crustacea and their distribution 
in depth at that lake. He finds that Cyclops is in September by 
far the most numerous genus, having a maximum number of over 
400,000 per square meter of surface. In August, however, he 
notes that only 140,000 per square meter was present. As -the 
opening of our dredge was -gV of a square meter, the average 
number of this genus caught by us during July would be at the 
rate of 50,000 per square meter and the maximum would be over 
136,000. Since the coefficient of the dredge can hardly be lower 
than four, the number of Cyclops per square meter of surface is 
not smaller in July in lake Mendota than that noted for August in 
the far-deeper lake Plon. No figures are given by Zacharias for 
Diaptomus , which was our most numerous species. Bosmina he 
finds abundant, while no pelagic Bosmina or Ceriodaphnioj has 
been found in lake Mendota. Hyalodaphnia is present in very 
large numbers, ranging from 62,000 to 94,000 per square m. 
A corresponding species of Hyalodaphnia is found in lake Men¬ 
dota in July and later in very small numbers. Only a few scat¬ 
tering individuals were noticed in our counting, the number being 
quite too small to use in the study of the Crustacea. 
In observations on the Crustacea from different depths, Zach¬ 
arias found the Crustacea distributed as follows: 
August 19. 
Hyalo¬ 
daphnia. 
Copepoda. 
Bosmina. 
Aug. 31 —Cyclops 
(2 obs ;. 
■0 — 10 m. 
540 
840 
150 
690 
690 
0 — 20 m. 
510 
1,350 
100 
2,190 
1,500 
0 —40m. 
660 
1,620 
100 
3,510 
1,320 
These figures indicate that almost or quite all of the indi¬ 
viduals of Daphnia and Bosmina were confined to the upper 10 m., 
and so far his observations agree substantially with our own. 
They indicate also that the Copepoda extend to very considerable 
