374 



UNUSUALLY TOLERANT SXAILS 



Musculium transversum, the leader of the water-breathing snails 

 in enduring low oxygen and other effects of pollution in upper Peoria 

 Lake, continued in the lower lake to still lower levels the decline begun 

 between Rome and Spring Bay in the summer of 1922 and showed no 

 increase worth mentioning over 1920 abundance (Table, p. 347). 

 The lower lake has recently had ordinarily a sufficient supply of oxy- 

 gen at levels well above the bottom for the coarser commercial fishes, 

 and is subject to little or no seining as a consequence of the almost 

 entire absence of good landing places. Perhaps the little-restricted 

 fish foraging may account in part for the small numbers recently found 

 not only of this species but also of others, including mud worms and 

 midge larvae, in the lower lake. A second circumstance that may 

 have bearing on the recent scarcity of this snail in lower Peoria Lake 

 is that in the period between 1913 and 1915, when bottom conditions 

 were more nearly normal in this part of the Illinois River, the upper 

 lake was, as in 1922, its principal stronghold. In those years the 

 lower lake snail fauna was largely made up of species of Viviparidae 

 and Pleuroceridae, though 71/. traiisz'crsiiin was also taken in some 

 hauls in moderate or small numbers. 



LESS TOLERANT SNAILS 



The less tolerant snails from lower lake hauls in the summer of 

 1928 were of only four kinds, as against ten kinds from the middle 

 lake the same season, but no more than three from the lower lake in 

 the summer of 1920. Three of these, Pmdinm coiiiprissuiii, P. pau- 

 perculum, var. crystalcnse, and Campcloma subsoliduni were present 

 both in upper and middle lake collections in 1932, while the fourth, 

 Spliacriiim staiuinciini. had its farthest northward 1922 occurrence in 

 the middle lake. All four of them were found in the Peoria Narrows 

 hauls, and three of them, or all excepting the variety of P. paupercu- 

 lum, were taken on the west beach side in the Workhouse Point cross- 

 section. Only the two of them least reputed previously for sensitive- 

 ness, P. coinprcssinn and Campcloin-a subsolidum, were found in the 

 channel or eastward of it below Peoria Narrows: and none of them 

 was taken at stations more than 500 feet to the eastward of the mid- 

 channel line or .south of the upper and cleaner third of the lower lake. 

 Numbers per collection or per unit of bottom area were small and 

 relatively unimportant in all cases. 



