526 
of plankton per m.’ In Novemberand December, however, the 
mean production in the tributary, for the only time in our rec- 
ords, exceeds by 67 and 7 per cent. the production in the chan- 
nel. On the average, however, production in the tributary, 
even in most favorable conditions, is less than in the main 
stream, and the contributions of the tributary continue to be 
generally a diluent of the channel plankton. 
If the means of the monthly averages are compared, the 
ratio, when all collections are included, between production in 
Spoon and Illinois rivers rises from 1 to 4.7 to 1 to 10.6. This 
ratio, which eliminates somewhat of the error resulting from 
differences in the number of collections in the two streams, 
probably represents more nearly the actual ratio of production 
in the two streams derivable from our data. 
In the table which follows, the catches in the two streams 
upon coincident—approximately so in five instances—dates 
only are averaged. This reduces the number of catches in the 
Illinois River from 235 to 33, modifies some of the monthly 
averages, and changes the ratios of production in the Illinois 
and Spoon rivers, based on means of the monthly averages, 
from 1 to 10.6 to 1 to 9.9, but does not otherwise materially 
alter the relationship of production in tributary and channel 
waters as determined on the basis of all collections. 
If the silt is not eliminated by estimation the average 
amount of the silk-net catch in the two streams is respectively 
1.41 and 3.98 cubic centimeters per cubic meter. The change 
in the ratio is due to the greater proportion of silt carried 
by Spoon River. 
Tn the table of plankton comparisons following page 342, 
the averages of the amount of plankton per cubic meter of 
water in the two streams is given by months for the years of 
our operations. Decimals to three places are not here indica- 
tive of great accuracy of measurement, They result from ef- 
forts to represent the small proportion which the plankton 
forms of the silt-laden catches from Spoon River. Considera- 
ble differences in these small quantities in this table and else- 
