488 THE AMERICAN NATURALIST.  [Vou. XXXIII. 
ing week the same peculiar distribution was observed, the sur- 
face water being free from them, while the number at the mid- 
depth was 336 percc. This arrangement continued until the 
first of September. During all this time not a single individual 
was observed at the surface, and the numbers at the mid-depth 
fluctuated between wide limits, on one day being as high as 
3640 per cc. At the bottom they were invariably present, but 
always in much smaller numbers than at the mid-depth. The 
following table gives the number of Mallomonas at the surface, 
mid-depth, and bottom for the entire period of growth. 
NuMBER PER CC. .NUMBER PER CC. 
Date. Sur. Mid. Bot. Date. Sur. Mid. Bot. 
June 18, o o o July 28, o 264 162 
24, o 116 42 Aug. 4, o ©3640 6 
July 1, o 336 2 Fiy 0 1200 I0 
5 o 4 18, O- 1380 4 
15, o 724 276 25; o 298 8 
22, o I102 158 Sept. L, o O o 
On July 17 a series of samples was taken at intervals of ten 
feet through the vertical, and the following results were 
obtained. 
DEPTH. MALLOMONAS PER CC. 
Surface. (0) 
10 ft. o 
15 ft. 2 
20 ft 1454 
25 ft. 794 
30 ft. 548 
40 ft. 112 
50 ft. 88 
60 ft. 64 
These figures, together with the temperature of the water at 
various depths, are shown by the diagram. It will be observed 
that the greatest numbers were found just below what may be 
called the thermocline. From the surface down to a depth of 
ten feet the temperature of the water was uniform. Between 
ten and twenty feet the temperature fell rapidly. The water in 
this stratum was not stagnant, however, as other observations — 
not given here — indicated. But below a depth of twenty feet 
