375 



While the comparison with variety and abundance in the same 

 group in the middle lake in 1922 was distinctly unfavorable, that with 

 the lower lake in the summer two years before, particularly in the 

 case of the Pisidia, was less so, and suggests moderate improvement 

 within the limited area of best conditions in the two years. Besides 

 taking in 1922 one more species than we did in 1920, one or the other 

 of the two species of that genus was found at five in all of the total 

 of eighteen stations visited in 1922; whereas in the summer of 1920 

 only a single species of the genus {P. comprcssuui) was taken, and it 

 was confined to a single station in the Peoria Narrows cross-section. 



No explanation of the reduction in variety in this group between 

 the middle and lower lakes recently is furnished by the comparison 

 with our 1913 and 1915 data. In those years we took as many kinds 

 and as great average numbers of Pisidium in the lower as in the middle 

 lake, and more kinds of Sphaerium in the lower lake than in either 

 the middle or upper; and found as well species of both genera at the 

 wide-water stations in the lower half of the lower lake, where they 

 were not taken at all either in 1922 or 1920. In view of these facts, 

 though based in some cases on few collections, it is difficult to avoid 

 the conclusion that local pollution has been injurious to this group in 

 lower Peoria Lake recently. 



LESS TOLERANT ASSOCIATED GROUP 



A group of seven miscellaneous species having at first sight no 

 characteristics in common except the fact that none of them was taken 

 above the lower lake cither in 1922 or 1920 completes the list of lower 

 lake bottom animals taken by us in the season of 1922. Of this group 

 three appeared in 1922 that were not found in collections in 1920, and 

 three inl920 that were absent in 1922, giving us seven representatives 

 of the lot in each summer's collections. The list included, as taken 

 either in 1922 or 1920, the following species, embracing insects other 

 than Chironomidac ; young mussels ; pleurocerid snails ; sponges ; bry- 

 ozoans ; and Crustacea : 



1922 only. 



Corixa species. — Peoria Narrows, in strong current. 



Lcptocerid (caddis) larva. — Peoria Narrows, in strong current. 



Anodonta iiiibeeillis .Say. — Mid-channel, opposite l-'ulton Street, 

 Pforia. in current. 



