404 



ment in the sanitary condition of the bottom muds. Tabulation shows, 

 however, for the four commoner species in the upper lake and for six in 

 the middle lake almost identical percentages of occurrence in both vears. 



It is necessary finally to add to the negative side of the accoinit in 

 Peoria Lake the still-continuing failure to reappear, anywhere, of several 

 of the most characteristic and, because of their large size as well as their 

 abundance in several instances, the most cons]ncuous members of the old 

 but now all but completely defunct gastrojwd snail fauna. Particular 

 nienlion should he made of FIvipara conlcctoidrs. I', siibl'itrpiirca. Liof'lax 

 siibrariiiiitiis. and two or more species of .\mnicolidae. Fuller lists of the 

 species which have disapjicared from Peoria Lake completely since some 

 time before \'.)20 are presented in jiages following. It is largely on the 

 basis of the failure of these snails to reapiiear in recent years that it h.is 



