350 



except in that respect from the abundant larva formerly referred to 

 C. fcntans in our work of ten years or so ago at and above Havana — - 

 a form which seems to have wholly disappeared from this section of 

 the Illinois River since some time before 1920. The practical confine- 

 ment of the more poUutional kinds of midge larvae recently to the lake 

 zones outside the channel, while to an extent perhaps a result of habitat 

 preference unconnected with pollution, places them conveniently in 

 those areas where the bottom receives the largest fresh supplies of rich 

 sediment as the gage recedes after floods. 



UNUSUALLY TOLER.^NT SX.MLS 



One of the most surprising facts that came out of the study of 

 the bottom dredgings from upper Peoria Lake was the enormous multi- 

 plication at Chillicothe and in the lake opposite Rome between 1920 

 and 1933 of the little sphaeriid snail, MuscuUum traiisz'crsiiiit Linsley. 

 Nuinbers of this small bivalve, familiar to local hunters and fishermen 

 under the name of "duck shell", reached a hundred thousand per square 

 yard in parts of the upper portion of the upper lake in the summer of 

 1923. Average numbers for all stations in cross-sections passed 51,000 

 at Chillicothe and 39,000 opposite Rome, these figures representing 

 equivalents of weight valuations between 19,000 and 25,000 pounds 

 per acre over limited areas, on the supposition' that all the yourfg and 

 half grown lived to become adult. The distribution of the largest 

 hauls of this little snail agreed closely with the location of the largest 

 catches of mud worms, and also an equally abrupt average decline in 

 the Musculium figures corresponded to the drop in worm figures al- 

 ready mentioned as having taken place in the two miles between the 

 Rome cross-section and the next one south. This decline was followed 

 by further decrease or showed little recovery in the areas sampled in 

 the middle and lower lakes. (Table, p. 347.) 



The ability of this snail to withstand low oxygen is well supported 

 by previous evidence from the Illinois River, where it occurred in 

 small numbers near the banks as far north as Marseilles in the sum- 

 mers of 1911 and 1912. Its recent extremely rapid nuiltiplication in 

 the worst instead of the best portion of its old range in Peoria Lake. 

 amounting to from 50 to more than 2,000 times numbers as of the 

 summer of 1920, may be supposed, however, to reflect quite probably 

 some other factor than those involved in the original habitat preferences 

 of the species. Perhaps it is true that an important consideration has 



