392 



since l'J2(), usually near shore cir in unusually good current, but where 

 all of the species in Group III have disappeared. 



Group III. Missing members of the old bottom fauna, as of our 

 lists of 1913-1915, including snails of the families Viviparidae and Amni- 

 colidae, chiefly ; insects or insect larvae or nymphs of the orders Ephe- 

 merida, Odonata, Hemiptera, Neuroptera, and Coleoptera ; and various 

 other small bottom animals. 



The word pollutional as here used corresponds roughly to Kolkwitz's 

 term "mesosaprobic" and to Forbes and Richardson's term "pollutional". 

 as used in the 1913 paper on the Illinois River. Both in its use and in 

 that of the word "tolerant" (rather than "contaminate") it has been my 

 purpose to attain greater flexibility than might be expected with the 

 customary restricted use of these or similar terms ; and by the use of the 

 latter term, more especially, to express better the breadth of range, merg- 

 ing preferences, and adaptability of the not inconsiderable group of 

 species which find their true position, quite unbounded by hard and fast 

 lines, almost anywhere between pollutional and strictly clean-water forms. 



SUJIM.\UV 



In the 44 miles of the Illinois River above Peoria Lake examined 

 in the summer of 1923 all of the bottom organisms taken belonged to 

 the more strictly pollutional or unusually tolerant kinds : the most of 

 them being small worms, at least one variety of which has previously 

 both in Illinois and elsewhere been found to be characteristic of septic 

 sludge. In this portion of the river the "sludge worms" were found to 

 be most abundant outside the channel, where there is greatest sedimen- 

 tation after floods. 



The small bottom animals taken in 11123 in the broadly expanded 

 20-mile section of river between Chillicothe and the foot of Peoria Lake 

 were with a few exceptions jiollutional or more or less tolerant forms, 

 in variety, abundance, and dispersal, showing no essential change since 

 the summer of 1922. 



Toward shore on the west or sand blutT side of the first 14 miles 

 of this 20-mile section, and in the faster current of the upper and lower 

 "narrows" or adjacent waters several less tolerant to fairly clean prefer- 

 ence species were taken, as they were also in 1920 and 1922: and these 

 showed some evidence of increase both in variety and numbers within 

 their recent range as thus localized. 



In about 71.5 miles of river below Peoria studied in 1923 enumera- 

 tions showed a clear though irregular <lccline in abundance of the tuhificid 

 worms as comj^ared with numbers above Peoria, as well as a decrease in 

 variety of other pollutional or unusually tolerant species. But when 

 the distribution of the collecting stations is taken into account this can 

 not be believed to signify much if anything more than that the niost of 

 this section of river has too hard a bottom or loo sharp local gradients to 

 furnish good lodgment and a "settled susiiended" food supply for these 



