379 



Of interest also is the apparent indication of a larger variety of 

 bottom species in the lower lake than in the upper or middle lake be- 

 fore 1920, and a correspondingly greater loss of kinds in the lower 

 lake species lists between 1915 and 1922. It is quite posisble that the 

 upper and middle lakes may have been injured to a minor extent by 

 sanitary canal pollution before 1915 and in advance of serious injury 

 to the lower lake from the same or local causes. But this can not 

 now be proven, and the continued existence of a healthy and varied 

 mussel fauna (see page 3S.-)) between Peoria Narrows and Chilli- 

 cothe as late as 1!)15, is against assuming that there had by that time 

 been any very serious change, at least in the channel, in or quite near 

 to which the mussel beds have always been located. 



The greater variety of habitats furnished by the lower lake and 

 its connections, already mentioned, may account for a good deal of 

 the difference shown ; and some of it may result from the greater num- 

 ber of lower lake bottom collections in the earlier jieriod, including 

 fuller special studies on the Chironomidae there than elsewhere, in 

 the course of short visits by Malloch to Peoria in liHli and 1914 ( Mal- 

 loch, 1915). But as si)ecial evidence suggesting earlier injury, par- 

 ticularly to the wide waters of both the upper and middle lakes, where 

 sedimentation after the spring floods is particularly heavy, may be 

 mentioned the noticeable absence or extreme rarity in the 191.3-1915 

 collections of several groups of insects; such as the commoner aquatic 

 Coleoptera ; various Odonata; and from the upper lake in particular, 

 the common May-fly nymph of the genus Hexagenia, as well as other 

 Ephemeridae. The limited amount of collecting in the vegetation 

 zones before 1920 also showed much poorer results than were easily 

 to be obtained at that time with the .same amount of effort in the lakes 

 possessing similar aquatic vegetation near Havana. 



COMMONER SMALL BOTTOM ANIMALS THAT HAVE 

 nrSAPPEAKED SINCE 1915 



While reviewing the missing members of the old fauna, it will be 

 recalled that the species obtained in Peoria Lake in 1922 and 1920 

 have already been arranged, in this and earlier ])apers in a descend- 

 ing order of tolerance without so far disclosing any that seem to have 

 a clear title as clean bottom species. The subjoined lists may be sup- 

 posed, on the contrary, to include a considerable number of kinds that 

 fall fairly under that designation, so far as they are applicable at all 



