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the seasonal distribution of individual species and these recurrent 
plant pulses will be discussed in connection with the various species 
wherever the data are available. For the present it will suffice to call 
attention to such correlation as exists between fluctuations of the 
phytoplankton and the total Plowma. The table on the preceding page 
gives the location and amplitude of the maxima of the ploiman pulses, 
and a graphic presentation of the seasonal curve of distribution of the 
total Rotifera will be found in Plates III. and IV. On comparison of 
the plorman pulses with those of the chlorophyll-bearing organisms, 
graphically presented in Plates I. and II., it will be found that 15 of 
the 33 pulses of Plotma contained within the period covered by the 
plates coincide in location with the plant pulses; that 12 follow at the 
next collection, usually a week later, and 3 within a fortnight; while 
only 3 of the 33 exhibit no such correlation. The data suggest 
strongly the agency of the plant pulses in building up the Ploima, 
and that the food relations are fundamental in the fluctuations of 
these planktonts. 
DISCUSSION OF SPECIES OF PLOIMA. 
Anurea aculeata Ehrbg.—Average number, 1,839. In 1898 
this species has a very well-defined and characteristic seasonal 
distribution (Table I.). It first appears March 8 at 37°, increases 
to a maximum of 45,200 on May 10 at 61°. then declines, and 
disappears June 14 at 83°. The curve of its occurrence in this year 
is a very symmetrical one. It reappears on December 27 at 32°, and 
there are scattered occurrences through the winter months of 1899. 
Records in other years suggest in the main a similar distribution. 
In 1896 it first appeared January 6, rose to a pulse of 6,550 on May 8 
at 76°, and, on the decline of the June rise, there was a second and 
larger pulse of 29,600 on June 17 at 76°. It reappeared on Decem- 
ber 29, and in 1897 reached a vernal maximum of 22,400 on May 25 
at 66°, then disappeared, and was not again noted in the following 
winter nor until March 8. In 1894 the last vernal record was made 
June 12, and on September 4, at 78°, there was an autumnal pulse 
of 13,825—a phenomenon not repeated in subsequent years. The 
normal course of its seasonal distribution in the river plankton seems 
to be as follows: reappearance in December when minimum 
temperatures have been reached; slow multiplication during the 
winter, and a well-defined pulse on the decline of the spring flood in 
(11) 
