537 
in the main stream. They are essentially littoral forms es- 
pecially common about driftwood, which abounds in Spoon 
River—a fact which doubtless accounts for their frequent oc- 
currence in the plankton of that stream. 
The total number of species of planktonts noted in the col- 
lections from the two streams in my enumerations of the 
plankton, and listed in Table XIV., is not to be taken as repre- 
senting the total number present. But a small part of each 
catch was examined, and rare species in both streams usually 
escaped detection. Their relation to each other, however, af- 
fords an index of the relative number of species present in dif- 
ferent collections in the same or different streams provided the 
method of examination is similar. The silt makes uniformity 
of dilution impossible in many cases, and thus introduces some 
error into the data. The average number of species noted in 
a Spoon River collection was 24 to 69 in the Illinois, a differ- 
ence less marked than that disclosed by the volumetric or the 
statistical comparison, but still significant of the relative 
paucity of the plankton of Spoon River and the more limited 
range of its constituent organisms. 
The total number of species recorded from Spoon River is 
170, while the Illinois yielded 429. The greater number of col- 
lections examined from the latter stream may explain a part 
of this difference. Without exception, all the identified species 
found in Spoon River have occurred also in the Illinois above 
the mouth of Spoon River. The only species in the tributary 
which seem to reach as great a development there as they do in 
the Illinois, or a still greater one, are the species of Surirella, 
especially S. splendida, several forms of Closterium, Cyclops 
serrulatus, and the dipterous larve above mentioned. 
A comparison of the total population of the two streams 
throughout the thirty months covered by our collections, as 
shown in the final columns of Table XIV., throws some further 
light on the relations between the plankton of the two streams. 
Traces of the same seasonal routine appear in both; the spring 
maximum appears in both at about the same time, while the 
