530 
a decline of 11 per cent. These figures are based on averages. 
Naturally in tributary floods the increased volume and les- 
sened plankton will cause a much greater dilution, though of 
brief duration. In moderate and low-water stages the decline 
will be less than the above average and of longer duration. 
The relative discharge of the two streams also enters to fur- 
ther complicate the problem. But whatever form the ratio of 
production of the two streams may assume in a precise determi- 
nation, it is safe to say that the general conclusion drawn 
from the present data will be confirmed. 
QUALITATIVE COMPARISON OF PLANKTON OF SPOON AND ILLINOIS RIVERS. 
The details of this subject, in so far as they pertain to the 
constituent species of the plankton of the two streams, will re- 
ceive attention in the discussion of species in the second part 
of this paper. For the present only the general phases and 
most striking contrasts will be discussed. 
With a view to presenting in concrete form the contrast in 
the relative population of thé two streams I have prepared 
Table XIV., which gives statistical data compiled from the 
enumeration records. The table exhibits the number of indi- 
viduals per cubic meter of water of the main groups of plank- 
tonts in the thirty-five collections in Spoon River, and in simi-- 
lar collections made on the same date in the Illinois River. In 
a few cases only, the Illinois River collections were made sey- 
eral days prior or subsequent to the Spoon River collection. 
The groups listed are the alge, diatoms, Rhizopoda, Masti- 
gophora, Infusoria, Rotatoria, Entomostraca, insect larve, and 
miscellaneous. The total numbers of individuals and of species 
are also given. These data are taken entirely from catches 
with the silk net, and are subject to the errors arising from 
leakage through the silk. The silt interferes in this case only 
in so far as it obscures the planktonts to a greater extent in 
Spoon River collections, and thus necessitates a greater dilution 
in counting and, in consequence, a greater factor in computa- 
tion. The margin of error is accordingly somewhat greater in 
