337 



2. The extension of range of the surviving Sphaeriidae between 

 1920 and 1923 went hand in hand with large expansion (following the 

 lull due to business depression of 1921) in the Packingtown slaughter- 

 ings at Chicago, and with tremendous increase in numbers of tubificid 

 worms in the nuids of tlie upper lake and the river at Chillicolhe ; and 

 suggests perhaps a progressive immunization of the already tolerant 

 sphaeriid snails to conditions which a few years earlier might have 

 gone harder with them. 



3. Both in the upper and the middle lake .in 1933 improvement in 

 the condition of the muds is indicated by increase in variety of the 

 species lists (1) as we proceed down stream; and (2) as we proceed 

 outward from mid-channel stations into the wide waters. 



4. In the lower lake there are various evidences of mixed con- 

 ditions on the bottom, partly due, in the wide waters, to wind-blown 

 local pollution ; and so far as suitable situations for a few cleaner- 

 preference forms are provided, apparently due to the much greater 

 current that prevails in the lower lake channel, or in the narrows (in 

 our results included as lake stations) immediately above or below it. 



5. Species that have definitely disappeared from Peoria Lake 

 since 1913-1915 include all but three or four kinds out of a total of 

 more than forty kinds of fresh-water mussels; all of the snails of the 

 family Aninicolidae ; all but one species of the formerly important and 

 especially conspicuous snail family Viviparidae ; and a varied weed 

 fauna. 



C. P'rom the rather restricted list of small bottom invertebrates 

 that seem to have been exterminated in Peoria Lake since 1913-1915, 

 compared with the decidedly larger lists from the vicinity of Havana*, 

 it is suggested that upper and middle Peoria Lake may have been 

 subjected to a measurable amount of injury from up-river pdllution 

 prior to 1915. 



• Sec Article VI of this volume, on 1"J23 fauna. 



