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 I., 

 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. I 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. 

 BACTERIACE^;.* 



Records were kept of the masses of the larger members of this 

 group which occurred in our plankton catches. They were princi- 

 pally the dichotomously branched brownish fragments of Creno- 

 thrix, filaments of Beggiatoa, and colonies of Micro coccus. The 

 average number recorded for this year was 57,142,822, and they 

 occur throughout the year in every collection, rarely falling below 



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