(Quadrula plicata, Pleuroccra elevatum, var. lewisii, Goniobasis livcs- 

 ceiis, Hyalella knickerbockeri, Plumatella princeps, var. fniticosa*, 

 and species closely allied in three other cases) were taken at stations in 

 the polluted sections of the upper Illinois where surface dissolved 

 oxygen ranged between 1.4 and 2.7 parts per million, and where the 

 great bulk of the normal bottom population had already been destroyed. 

 One of the .snails, also, Pleuroccra elcvatum, was one of the only two 

 snails taken by Jewell in the acid waters of the Big Muddy River in 

 southern Illinois in 1919 (Jewell, 1922). 



Comparison of the Recent Small Invertebrate Bottom 



and Weed Fauna of Peoria Lake with that of 



1913-1915. 



GENERAL COMPARISON 



Pre-] 930 records of the bottom fauna, including the mussels, from 

 the section of the Illinois River covered in the present paper were 

 made by us at Chillicothe in the autumn of 1911 and the sinnmer of 

 1912; and collections of the smaller bottom species only were made 

 in a series of cross-sections between Chillicothe and the foot of lower 

 Peoria Lake in the summers of 1913 and 1914 (Forbes and Richard- 

 son, 1913 : Richardson, 1921). In addition to this earlier work of our 

 own, the United States Bureau of Fisheries made a survey of the 

 mussel resources of Peoria Lake, in connection with one of the entire 

 Illinois River, during the years 1911 and 1912 (Danglade, 1914). 



In making comparisons with these earlier records it is not possible 

 always to be as exact as one could wish, because determinations were 

 not carried out in as much detail in the earlier period as has been 

 the case with more recent collections. For this reason, and also be- 

 cause of the necessarily very small fraction of the total bottom area 

 covered by dredge hauls, neither our lists of the earlier nor the more 

 recent bottom species of the lake are assumed to be as complete as they 

 might have been with heavier programs of dredgings. Previous ex- 

 perience in dredging in the Illinois River and in other wa'.ers in Illi- 

 nois has shown, however, that the number of bottom species that may 

 be expected in single hauls is never very large, apparently as a con- 

 sequence of a general rule that one or two or at the best a few species 

 tend to iiredominate, usually with great distinctness, in a given range. 



• At that time Incorrectly Identified. 



