Article V'I. — The Illinois Riz'cr Bottom Faiiiia in /p-'j?. Bv Robert 

 E. Richardson. 



The i)resent paper is based on one hundred and fifty Petersen 

 sampler collections taken in the Illinois River and Peoria Lake between 

 June and September 1923 ; includinj^- •H> taken between La Salle and the 

 head of Peoria Lake; 75 in the lake or its entrance, outlet, or connecting- 

 '"narrows" ; and 40 below the lake and between Peoria and Beardstown. 



The subdivision of the river and lake into zones follows that in the 

 article on the 1922 bottom fauna as respects Peoria Lake, emphasizing 

 distance from the channel center rather than depth, and recognizing three 

 subdivisions as follows: (1) a central, imaginary, channel zone of about 

 700 feet average total width; (2) to either side and adjacent to this an 

 intermediate zone extending outwand to the 1500 foot line, on either sitle 

 of the channel center; and outside of this, where the width permits. (:?) 

 an outer zone extending anywhere from somewhat more than 1500 to 

 more than -1800 feet beyond the mid-channel line at recent summer levels 

 with normal rainfall. In the river the so-called intermediate and outer 

 zones corres])ond for the most part, except where the river is very shal- 

 low, to the zones of 4 to 7 and 1 to 3 feet as used in preceding papers, 

 reference being to recent normal summer levels. 



The subdivision of the species on the basis of degree of tolerance 

 also follows in main features that of the 1923 paper, which recognizes, 

 in general, three greater groups, as follows ; 



Group I. Pollutional or more or less tolerant species, including all 

 recent tubificid worms, leeches, and midge larvae of Peoria Lake ; and a 

 number of more than ordinarily tolerant snails belonging chiefly to the 

 sphaeriid genera Musculium and Pisidium. The basis of inclusion in 

 this group has been chiefly the fact of survival in Peoria Lake through 

 1920, a short time before which year a sizable list of the more .sensitive 

 of the members of the old bottom population seem to have been wholly 

 exterminated. P'or the distinction between the words pollutional and 

 tolerant as used above and elsewhere in this ])aper, and the general align- 

 ment of terminoolgy with that of our 191;! paper on the biology of the 

 Upper Illinois*, see the paper on changes in the bottom fauna of Peoria 

 Lake, 1920 to 1922t. 



Group II. Cleaner preference species, including principally current- 

 loving forms of snails, Bryozoa. insects, and Crustacea, that have per- 

 sisted in Peoria Lake or in the river shortly southward through and. 



• But. III. state Kab. Nat. Hist.. Vol. IX. Art. 10. .Tune. I91.'!. 

 t Hill III. .'^tale .Vat. Hist. Siirv.. Vol. XV. Art. V. 1!I25. 



