448 Birge—Vertical Distribution of Pelagic Crustacea. 
The greatest variations in successive hauls were: 
July 27. 
July 19. 
July 27. 
6 P. M.3759 
9 P. M.7134 
12 Midnight.. ......... .3898 
3 A. M...4564 
6 A. M.. .....5478 
3 A. M ...4021 
6 A. M...8703 
The last is the only case where one catch is double the 
preceding one. 
Averaging the total catch of the periods, we find: 
Period I...........3808 
Period II .......4253 
Period III ..... .5144 
Period IV.. ....... .3428 
In spite of this absolute increase in numbers the percentage of 
Diaptomus to the whole number of Crustacea declined, chiefly on 
account of the more rapid increase of Cyclops . In Period I 
Diaptomus constituted 79 per cent, of the Crustacea; in Period 
II 66. B per cent; in Period III 57.1 percent. ; in Per.odIV 53.3 
per cent. 
These numbers would indicate that the period of maximum 
number of Diaptomus falls in July. This conclusion is confirmed 
by subsequent observations. Catches made on August 24 and 
25, 3 in number, gave from 1000 to 1500 Diaptomi. After Sep¬ 
tember 18, when regular observations were resumed, very few 
catches passed 1000, and in October the number declined still 
more. 
A curious fact, for which no full explanation is possible, is 
that the catches made by night average .smaller than those by 
day. No cause can be assigned for this at present beyond 
chance. All but one of the exceptionally large catches occurred 
during the day, and although thq smallest catch was made at 6 
a. m., most of the exceptionally small ones were made at 
night. The night catches of Daphnia hyalina average some¬ 
what larger than those of the day. 
Distribution in depth .—As has already been stated, the water 
of the lake at the point of observation was slightly over 18 m. 
in depth, so that it was possible to obtain material from six 
levels of 3 m. each. In the lowest level, that from 15 m. to 
