ARTICLE I.—Plankton Studies. V.' The Plankton of the Ill- 
nois River, 1894-1899. Part II. Constituent Organisms and their 
Seasonal Distribution. By C. A. Kororp. 

INTRODUCTION. 
This paper gives the results of a statistical study of a series of 
quantitative plankton collections made in the channel of the Hlinois 
River near Havana, IIl., at the Illinois Biological Station, in 1894— 
1899. The environmental conditions and the volumetric results of 
this investigation have been given in Part I. (Kofoid,’03), published 
in Volume VI. of this Bulletin. 
Of the 235 collections made in channel waters and used in the 
quantitative study, only 182 were subjected to numerical and 
qualitative analysis. The omitted collections were intercalated at 
brief intervals of one to several days between those enumerated, 
principally in the summer of 1895 and during the winter flood of 
1896 and the summer of the same year. The collections chosen for 
this study, whenever possible, represent a weekly interval, and a 
full list of all collections, with environmental data, may be found in 
Table III. of Part I. The chronological distribution of the collec- 
tions studied by the statistical method is. given in the table on 
the following page. 
The work of enumeration and the primary tabulation was com- 
pleted at Urbana December 31, 1900, when my formal connection 
with the State Laboratory ceased. The manuscript has been 

1The four preceding numbers of this series, all by the present writer, have 
been published as articles of the Bulletin of the Illinois State Laboratory of 
Natural History, as follows :— 
Article I., Vol. V.i—Plankton Studies. I. Methods and Apparatus in Use in 
Plankton Investigations at the Biological Experiment Station of the University of 
Illinois. 
Article V., Vol. V.—Plankton Studies. II. On Pleodorina illinoisensis, a New 
Species from the Plankton of the Illinois River. 
Article [X., Vol. V—Plankton Studies. III. On Platydorina, a New Genus 
of the Family Volvocide, from the Plankton of the Illinois River. 
Article II., Vol. VI.—Plankton Studies. IV. The Plankton of the Illinois 
River, 1894-1899, with Introductory Notes upon the Hydrography of the Illinois 
River and its Basin. Part I. Quantitative Investigations and General Results. 
(2) 
