450 Birge— Vertical Distribution of Pelagic Crustacea. 
The numbers vary from 984 to 4212, with an average for the 
whole month of 2229. The next level (3 — 6 m.) contained 
on the average 29.27 per cent, of Diaptomus. The maximum 
was 48.6 per cent. (J. 16, 12 m.), the minimum 16.4 (J. 17, 
6 p. m.). The average number was 1257, the minimum 462, the 
maximum 3244. This level showed on the whole the smallest 
amount of variation. 
The 6—9 m. level varied greatly, as would be expected, since 
it contains the lower part of the densely populated region. Its 
average content was 14 per cent, of the catch, the maximum 
32.5 per cent. (J. 27, 12 midnight), the minimum 3.4 (J. 17, 
6 p. m.). It is noteworthy that in the second and third periods 
the percentage found in this level at night was far larger than 
in the day, the day average in the second period being 4.8 per 
cent., while that at night was 17.6 per cent. In the third 
period the numbers were 12.2 per cent, and 22.9 per cent, re¬ 
spectively. In the fourth period the difference was very slight— 
practically nothing, and in the first the day catch was slightly 
larger in percentage, though almost exactly the same in numbers. 
The center of population for Diaptomus , that is the level 
above which 50 per cent, of the animals are found, lay on the 
average just above the 3 m. level, if we assume a uniform dis¬ 
tribution of the animals in each level. It varied from a depth 
of about 2.4 meters in Period II, to 3.6 m. in Period III, and 
3.3 m. in Period IV. The depth seems to depend upon the de¬ 
gree of disturbance of the water. In calm water the Crustacea 
aggregated nearer the surface, and had a wider distribution in 
stormy weather when the water was disturbed. 
Diurnal distribution .—It will be seen from the accompanying 
tables and plates that no noteworthy difference exists between 
the vertical distribution of Diaptomus by day and by night. The 
averages of the observations for each period show that there 
was no tendency of the Diaptomi toward the surface by night, 
or sinking by day. A closer study of the detailed results serves 
to strengthen the conclusions drawn from the average. No 
single observation shows any tendency toward a vertical move¬ 
ment in this direction. In the middle level (3-6 m.) the day and 
night observations are close together—practically identical— 
