410 



Changes in Number of Group II (cleaner preference) 



Species betwekn 1920 and 1923 



all zones combined 



* All from lower lake. 

 t Liverpool to Havana. 

 t Matanzas to head of Hickory Lake. 



The Missing Small Bottom Animals of the 1913-1915 Species 



Lists, apparently Exterminated some Time between 



1915 and 1920. Head of Peoria Lake to Beardstown 



To any one familiar witli the biology of the Illinois River between 

 1010 and 1983 a more striking change in the period than even the in- 

 trusion of poUutional forms and their increase has been the complete 

 disappearance in the nearly ninety miles between the head of Peoria 

 Lake and Beardstown of a long list of the more sensitive snails and 

 associated bottom animals in the less than ten year period between 1910 

 and 1920. In making comparisons of the more recent species lists with 

 those of 1913-1913, allowance need? to be made for some inequalities in 

 luniiber and distribution of collections, as well as in completeness of de- 

 termination of material, in the two periods. The total number of dredge 

 hauls made between Chillicothe and Beardstown in the seasons 1913-191.5 

 was 210, distributed quite evenly, as will be seen in table, p. 413, between 

 the four short reaches distinguished in this distance. In the three years 

 of collecting between 1930 and 1923 there were taken in all 2G6 collec- 

 tions in substantially the same areas and linear distance, but with much 



