368 



number from ten kinds to only four kinds of the less tolerant snails, 

 at the same time with the first appearance below Chillicothe of seven 

 miscellaneous small botloni animals of presumably lesser tolerance 

 than any of the groups taken either in 1922 or 1920 in the middle or 

 the upper lake. 



The comparison of 1922 collections from the lower lake with 

 those of 1920 (Table on p. 3?0) showed less dicerence than app>eared 

 either in the upper or middle lake, and suggests that this section of 

 the lake, with its greater average current, and its long histor)- of local 

 pollution, has at the present time a small bottom population somewhat 

 better adjusted to the prevailing conditions than has been the case 

 recently in the portions of the lake above Peoria Narrows. 



As we would expect from the heavy local pollution adjacent to 

 the channel on the east side, there was a greater increase in variety 

 in the species lists from the channel outward into the wide waters 

 (from 2 kinds to 12 kinds in the Workhouse Point cross-section; and 

 from 8 kinds to 17 kinds in the Fulton Street section) than either in 

 the middle or the upper lake (Table on p. 372). This was despite the 

 fact that the corrective action of the more rapid current in and also 

 well outside of the channel here permits the existence of several cur- 

 rent-loving forms that have not been found since- 1015 above the lower 

 lake ; and that wind- or wave-carried local pollution has apparently 

 reduced the number of kinds of the less tolerant Sphaeriidae in the 

 wide waters. 



The changes in the lower lake species lists down stream (Table 

 on p. 371) were irregular and of no special significance. 



