351 



been that this snail has recently met almost no hindrance to multipli- 

 cation from its greatest predatory enemy, the carp, in this poorly 

 aerated section of Peoria Lake. The supposition that its recent dis- 

 tribution and abundance has been in substance inverse to that of the 

 coarse bottom-feeding fishes is borne out by such information as we 

 have concerning the location of the most profitable seine hauls recently 

 above Peoria Narrows. It may also be the case that there has re- 

 cently been some increase in an already relatively strong innnunity in 

 this species to the ill efiects of low oxygen. 



LESS TOLERANT SNAILS 



It was in the group of less tolerant snails that the greatest increase 

 in variety of the small bottom fauna of the upper lake occurred in the 

 two years following 1920. The species included under this designa- 

 tion are all evidently less tolerant than MhschUuiii transvcrsnm, as 

 shown both by their greater rarity in upper Peoria Lake in 1922, and 

 by the fact that none of them occurred quite so far north in the Illi- 

 nois River in 1912 as that very tolerant species. Though less tolerant 

 than that unusually hardy form, they are all evidently considerably 

 more so than several other snail species (particularly V hi para contec- 

 toidcs Binney, V. suhpnrpurca Say, Lioplax subcarinatus Say, and 

 Anmicola emarginata Kiister), all of which were common in Peoria 

 Lake up to 1915 but have not since appeared in collections from either 

 the upper, middle, or lower lake. Though no representative of this 

 group could be called abundant at any upper lake station in 1922, we 

 took then, in all, five species, all but one belonging to the Sphaeriidae, 

 compared with a single S])ecies in 1920. The channel collections con- 

 tributed two species, MusciiUkiii tnntcatum Linsley and Pisidiitni coiii- 

 prcssiim Prime, against none at all in 1920; and the collections from 

 the open lake five kinds, M. truticatum, P. compressum, P. paupcrcn- 

 liiw var. crystalcnse Sterki, a species of Pisidium near P. complanalitm 

 Sterki, and Caiiipcloimi subsolidiiui Anthony, as against the last species 

 alone in the summer of 1920. 



Of these five snails, only one, the large viviparid, C. subsolidttin, 

 has been previously known through published records, so far as we 

 know, to be much more than ordinarily resistant to pollution or low 

 oxygen. This snail was taken by us in the Illinois River in 1912 as 

 far north as Starved Rock, or farther north than any other snail excejjt 

 Mitscniiiiiii transvcrstiiit ; and was recently reported by Miss Jewell 



