336 



Table op Mile Numbers and Distances, Peoria Lake, 1922 



• Petevscn dredge cross-section. 1922. 



Summary 



1. Improvement in both tlie upper and middle lake since 1920 is 

 indicated by the close to doubling of the species lists of both lakes in 

 the two years ending summer 1922 ; but the increase in kinds was 

 almost wholly confined to the group of tolerant sphaeriid snails, which 

 seem to have survived the mortality (between 1915 and 1918) that 

 destroyed practically all the other snail families, and between 1920 and 

 1922 to have again spread sufficiently over their old range to appear 

 more frequently than in the first-named year in dredge hauls. 



