409 



is noted, on the other hand, in the Hniited reach of around nineteen 

 miles between the Spring Lake Canal and Havana, .where in 192;) twelve 

 cleaner-preference forms were taken in nine dredge hauls compared 

 with only two in fourteen hauls in 1920, a finding that would seem at 

 least on its face to indicate some improvement in the condition of the 

 muds in the three years. The number of group II species taken below- 

 Havana was, however, only half as many in 1923 as in 1920, in a mileage 

 considerablv greater, though with the number of collections less, run- 

 ning to a total of only six species in thirteen collections covering 

 thirty miles of river length in 1923, compared with twelve kinds in 

 twenty-two collections from about 19 miles of river in the summer three 

 years before. As already mentioned, the section of more or less shifting 

 sand or sand and shell to hard bottom between Havana and Reardstown 

 was known before the recent mortality for the sparseness and irregularity 

 of distribution of its small bottom poi)ulation ; and because of this and 

 the factors favorable to error that lie back of it, less importance is prob- 

 ably to be attached to such variations in numbers than in sections where 

 the bottom-dwelling organisms — chiefly mud bottom forms in the Illinois 

 River — have a more suitable and more stable substratum on wdiich to 

 live. A better test of the changes that have taken place in recent years 

 in this and other sections of the Illinois River, inclusive of Peoria 

 Lake, because it concerns both larger numbers and greater and therefore 

 more convincing contrasts, consists in g'jing farther back, to the period 

 1913-1915. comparing our recent species lists with those of that time, 

 and noting the absentee names of the old fauna, side by side with recent 

 survivals and new entrants. 



