405 



been possible to distinguish the present and recent sphaeriid snail fauna 

 of the lake and ujjper river as belonging to a distinctly more tolerant 

 grouping than any except possibly one (Cainpcloiiui subsolidinn) of the 

 Illinois River Gastropoda. 



Illinois River below Peoria, 

 P. P. U. Bridge— Beardstown, 71.5 Miles 



Some imjM'ovement is indicated as we ))rocccd down the river from 

 Peoria, it appears, both by the lessening variety and abundance of the 

 tubilicid worms and the more ])ollutioiTal midge larvae; tables, pp 407-408. 

 The large decreases in the total of group I — polliilional and more or less 

 tolerant species — from 34-35 kinds in Peoria Lake to just over a dozen 

 kinds in the two reaches of river recognized above and below Havana, 

 do not, however, mean at all what a first glance at the figures might 

 indicate. The far greater ])ortion of the decrease in the number of group 

 I species is in fact due to the dropping out below Peoria Lake of most of 

 the less tolerant S])haeriid snails and Chironomidae ; a matter without 

 doubt due largely t(j the lesser suitability of the generally harder mud or 

 sand and shell bottom of the river in these sections (if we excejjt the first 

 S miles above Hanava ) than to any other cause; the species of Pisidium 

 and Sphaerium, in particular, which have persisted through the recent 

 pollution in Peoria Lake, never having been found either in variety or 

 numbers in the river between Peoria and Beardstown in the past fomleen 

 years of collecting. 



So far as the'grou]) II or cleaner-i)reference species are concerned, 

 again, instead of increasing down stream below Peoria as the grou]> I 

 kinds decrease, as we would expect if the latter indicated im]irovement, 

 they also fall oflf rapidly in variety as we proceed south, though, it is to 

 be noted, along with a rather marked decrease also in the number of col- 

 lections ; table, \>. UKi. The greatest variety of grouji II kinds in the three' 

 reaches recognized below Peoria Lake was taken in \.\\^ somewhat more 

 than 20 miles between Wesley and the Copperas Creek Dam, where IS 

 group II species were taken in \\yi'A in 21 collections, comjjaring with 20 

 in Peoria Lake from 75 collections the same season. Here there is for 

 the most ]iart good current, and princii)ally harder bottom than in Peoria 

 Lake, both of which indicate reduced sedimentation, which seems to pro- 

 tect a good many current-loving organisms, such as Planaria, Bryozoa of 

 .several kinds, the larvae of certain Trichojjtera (caddis-flies), and others, 

 which might otherwise succumb in the face of the new load of Peoria and 

 Pekin ))oilution. Between the inouth of the S])ring Lake Canal, shortly 

 below Copperas Creek Dam, and Havana, the number of cleaner-prefer- 

 ence s|)ecies taken dio]iped to only twelve kinds in the total of nine collec- 

 tions taken; a number perhajis not unduly small, in view both of the con- 

 dition of the mud and the small number f)f collections; though it is recalled 

 that this was by all odds the richest section along the whole Illinois River 

 some ten years ago, at which time it yielded nearly seventy kinds of small 



