The Rhizopoda are twelve times as abundant in the Illinois 
as in Spoon River. During the winter and spring flood of 1597 
the tributary waters contained more Rhizopoda than the main 
stream, but throughout the remaining period of our collections 
the tributary waters would serve, almost as a rule, as diluents of 
the rhizopodan fauna of the Illinois. This group of planktonts 
did not show an increase in the low-water period of 1897 simi- 
lar to that of many other groups—a phenomenon to be ex- 
plained by the fact that the diatoms, their main food supply, 
made no unusual growth during that period. 
The species of Rhizopoda are about half as many in Spoon 
River as in the Illinois (17 to 30—see table on page 518), and 
all of the species from Spoon River are present in much the 
same relative proportions in the Illinois. There do not appear 
to be any which find their centers of distribution in the tribu- 
tary. Its contributions of rhizopods thus neither diversify nor 
relatively increase the plankton in the main stream. 
The data concerning Mastigophora are to a large extent 
vitiated by the error of leakage. The tabulations of the silk- 
net catches indicate their great predominance in the main 
stream, 1 to 24 as an average, though other seasons than the 
low water of 1897 exhibit a much greater contrast, that for the 
whole year 1895, for example, being 1 to 351. In the period of 
low water of 1897 the tributary stream showed a greater devel- 
opment of these organisms than the main stream, especially of 
the green forms. 
The presence of these organisms in Spoon River in consid- 
erable numbers during the heated term of every year is indi- 
cated by the remarkable water-bloom which appears on clear 
still days about the middle of the afternoon. This is similar 
to that of the Illinois, but seems to be better developed, taking 
the form of a decided green scum on the surface. How far 
this indicates a quantitative predominance of the organisms of 
the water-bloom can only be determined when our filter 
catches shall have been examined. So far as the silk catches 
go they do not show the quantitative predominance of the 
