21 
averaging about 200,000,000, and reaching 1,697,000,000 August 9, 
1898. The temperatures during these pulses are above 60°, and 
the period of the maximum comes toward the close of that of max- 
imum summer temperatures, and sometimes in the autumn decline 
(Pt. I., Pl. XI. and XII.), when low and often stable river-levels 
usually prevail. A vernal and an early autumnal pulse are thus 
both present in the distribution of this species. It is not improb- 
able that other species than the one named have been included in 
the enumeration along with it on account of the small size and lack 
of striking characteristics. There are suggestions of recurrent 
pulses at intervals of 2-6 weeks in the records (Table I.). 
Oscillatoria spp.—Average number, 15,431 (filter-paper, 637,692). 
Fhe probable inclusion of several species in the sums under this 
heading may account in part for the irregularity of the seasonal 
curve. Oscillatoria has appeared in every month of the year, 
though the occurrences were most frequent in the period from July 
till the first of October. The numbers are exceedingly irregular 
and variable, and the pulses of numbers seem to attend the initial 
stage of floods following stable conditions. Thus, while these 
organisms occurred but singly or sparingly in the plankton during 
the autumn of 1897, they rose to 277,200 with the flood of January 
11, 1898, doubtless torn loose by the current from the bottom— 
their normal habitat. They are thus usually adventitious addi- 
tions to the plankton. Their frequent irruption into the plankton 
during midsummer and early autumn, and to some extent at other 
times, is due in part to the evolution of marsh gas in the detritus on 
the bottom. This breaks up the mats of Oscillatorta which coat 
the bottom and distributes them through the upper levels, where 
they remain in suspension for some time. This phenomenon is 
more prevalent in the marshy backwaters than it is in the river. 
Flood invasion in midsummer into the backwaters, such as Quiver 
Lake, is wont to cause there stagnation and great increase in Oscil- 
latoria, which to some extent enters the river with the run-off of the 
flood. Movements in the water and the evolution of marsh gas are 
thus principally responsible for the presence of Oscillatoria in the 
plankton. It still remains possible that its flotation during periods 
of optimum conditions of growth may be due to internal physio- 
logical conditions which lower the specific gravity of the organism. 
- Its great abundance at times in upper levels in the backwaters sug- 
