38 
Even the surviving snails showed in 1920 great reduction in average 
numbers and weight since 1915 in nearly all portions of the area studied. 
The extent to which they had shrunken in the different depth-zones of 
the upper, middle, and lower lake is shown in the comparative valuation 
tables appended, and it will be sufficient here to note that whereas in 1915 
the weight of the combined Gastropoda and Sphaeriidae in Peoria Lake 
collections made up from over 85 to over 99 per cent. of the total edible 
matter in the average haul, the snails in 1920, except in the short swift 
stretch through Peoria Narrows and in a few collections in the channel 
of the lower lake, weighed on the average less or only a little more than 
did the sludge-worms or midge larvae, and made up, in fact, only from 2 
to 15 per cent. of the average catch under the new dispensation. 
The Viviparidae and Pleuroceridae in 1920 were found only in the 
channel and 4—7-foot zone of the lower lake, opposite Peoria, and in the 
1—8-foot zone, on the east side in the Rome—Mossville section. The 
single surviving species of Sphaeriidae (Musculium transversum) showed 
a visibly wider distribution than the large Gastropoda, and it was some- 
what of a surprise to us to take the largest hauls of this stoutly tolerant 
little shell among the sludge-worms and larval midges of the filthy chan- 
nel-bottom between Chillicothe and the foot of the upper lake. 
Snatz Fauna or Borrom Mups, CHILLICOTHE TO Foor or PEORIA LAKE, 
1915 anp 1920 
1. Average number per square yard, main groups 
Channel 4-7-ft. zone | 1-3-ft. zone 
1915 1920 | 1915 1920 | 1915 1920 
wed —-- 3 
Upper lake 111 0 32 0 10 24 
Viviparidae ? 
and Middle lake 19 0 28 Om) 7 0 
Pleuroceridae 
Lower lake 134 63 153 8 50 0 
Upper lake 152 112 204 24 12 24 
Sphaeriidae Middle lake 91 0 116 3 277 12 
Lower lake 5 6 97 0 30 0 
Upper lake 6 0 3 0 32 0 
Amnicolidae, |Tower lake 0 0: ie ae 0 12 0 
Valvatidae, 
ce Middle lake 0 0 70 0 40 0 
i 
