357 



Compared with 1!)20, there was substantially a doubling in var- 

 iety of the 1922 lists, whether in channel, wide waters, or all zones 

 combined (Table 2, p. 358). The increases over 1!)20 were principally 

 due to multiplication of kinds of the less tolerant snails, which rose in 

 the two years from one to ten species ; and to the less tolerant kinds 

 of larval midges, which increased from two kinds to five. Both the 

 snails and the less tolerant midges showed the greatest gain in the 

 wide waters, at the same time that the more pollutional kinds of midges 

 fell off in representation in the extra-channel areas as compared with 

 1920. 



The changes in the species lists of the middle lake from its upper 

 to its low-er end in 1922 were irregular in the channel, the Al Fresco 

 Park stations showing scarcely better than those opposite Mossville, 

 four and a half iiiiles above (Table on p. 359). The largest variety of 

 channel species was found in the intermediate cross-section, opposite 

 the foot of Horshor Island, and about half way between the head and 

 the foot of the lake. In the wide waters there was a more regular 

 showing of improvement southward from Mossville, both the Horshor 

 Island and the Al Fresco cross-sections yielding nearly or quite twice 

 as many of the less tolerant snails and from three to four times as 

 man}' of the less tolerant midges as were taken opposite the Mossville 

 station. 



In all three of the middle lake cross-sections of 1922 there was 

 marked increase in variety of species as we proceeded outward into 

 the wide waters from the steamboat channel (Table on p. 360). This 

 improvement was mainly due in all three cross-sections to the increase 

 in number of kinds of less tolerant snails ; which rose in the Moss- 

 ville section from none in the channel to three kinds in the wide waters ; 

 in the Horshor Island section from three kinds in the channel to six 

 kinds in the wide waters ; and in the Al Fresco Park section from one 

 kind in the channel to five kinds at the extra-channel stations. 



