1922 and 1920. 



Goniobasis livescens Menke. — P. P. U. Bridge, in strong current. 



Sponge species. — Peoria Narrows, in strong current, 1920; op- 

 posite Fulton Street, Peoria, 1922, 600 feet east of mid-channel, in 

 current greater than that outside of channel in middle or upper lake. 



Hyalella knickerbockeri (Bate). — In or near channel, several sta- 

 tions, both 1923 and 1920, in current; once in far wide-waters, 1920. 

 near edge of smartweed bed. 



Plumatclla pr'mccps var. fniticosa Kraepelin. — Peoria Narrows 

 and P. P.. U. Bridge in strong curent, and just outside of channel op- 

 posite Liberty Street, 1920; mid-channel and 600 feet east, opposite 

 Fulton Street, 1922. 

 1920 only. 



Quadrula plicata Say, young. — Peoria Narrows only, in strong 

 current. 



Plcuroccra clcvatuiu var. Iczi'isii Lea. — Peoria Narrows and P. P. 

 U. Bridge, in strong current. 



Hydropsyche species. — Peoria Narrows and P. P. U. Bridge, a 

 strong current. 



Evidently an important consideration about this otherwise motley 

 group is that ail but one of them were confined both in 1922 and 1920 

 either to the swifter water at Peoria Narrows or the P. P. U. Rail- 

 way Bridge, slightly above and below the lake proper ; or were con- 

 fined if taken in open lake cross-sections to the first 600 feet eastward 

 of the mid-channel line, where also the current is appreciably greater 

 than in corresponding locations in the upper and middle lakes. Be- 

 sides this, the little fresh-water shrimp, Hyalella, which was found 

 once in the east wide-waters near the edge of a smartweed bed, as 

 also the species of Corixa, are really not bottom forms in the strict 

 sense, being both free swimmers to a considerable extent and. in the 

 case of the corixids, coming to the surface occasionally for air. 



In favor, on the other hand, of a higher degree of tolerance in 

 all members of this group than that jiossessed b)- the recently extermi- 

 nated mussels, snails, and other bottom species of all three lakes is 

 the fact that they have survived in recent years where so many other 

 species have perished. Such a supposition receives support from our 

 records from the polluted waters of the upper Illinois River obtained 

 in the years 1911 and 1912 (Forbes and Richardson, 1913). At that 

 time the identical species in the case of five of the 1920-1922 list 



