DISCUSSION OF THE STATISTICAL DATA OF THE SPECIES COMPOSING 
THE PLANKTON OF THE ILLINOIS RIVER IN 1894-1899. 
In the following pages the organisms occurring in the plankton 
of the Illinois River will be recorded, and from the statistical 
data accumulated by the enumeration method, facts pertaining to 
their relative abundance, seasonal distribution, and periods of max- 
imum occurrence will be cited. The average number per cubic 
meter for the year 1898 will be given, based upon the averages of 
52 collections distributed regularly throughout the year (Part L., 
Table III.). This year is chosen because of the regularity of the 
times of collection and the absence of any considerable irregularity 
in the hydrograph. Statements concerning seasonal distribution, 
etc., are based upon the records for all the years—1894-1899. All 
figures pertaining to species or groups marked with an asterisk, 
and starred figures elsewhere, are based upon filter-paper catches; 
all others, upon those of the silk net. Temperatures are in Fahren- 
heit, and are of surface waters at time of collection. 
The margin of error in statistical work of this sort is confessedly 
large. The complex character of the data with which I am deal- 
ing, and especially the extreme range in numbers, have made it 
necessary that I should adopt some consistent method of treating 
the computations. 1 have therefore chosen to carry out the num- 
bers to units, as the most feasible method of avoiding ‘confusion in 
the handling of the data. The use of round numbers would have 
been just as accurate. Computation to units is therefore to be 
understood as a matter of convenience, and not as an effort to 
exhibit a false and unattainable accuracy. 
CRYPTOGAMIA. 
BACTERIACEZ.* 
Records were kept of the masses of the larger members of this 
group which occurred in our plankton catches. They were princ’- 
pally the dichotomously branched brownish fragments of Creno- 
thrix, filaments of Beggiatoa, and colonies of Micrococcus. The 
average number recorded for this year was 57,142,822, aud they 
occur throughout the year in every collection, rarely falling below 
18 
