414 
weight running in these reaches from 86 to over 99%, and falling below 
90% only in the 1—3-foot zone above the Copperas Creek dam. Below 
the Lagrange dam, where the large Ephemeridae (May-flies) were rela- 
tively much more abundant than farther north, the Mollusca percentages 
dropper to an average range between 35 and 68%. 
In the sections above Lagrange, if we except the 4—7-foot zone 
between Copperas Creek dam and Havana, the larger snails ( Viviparidae 
and Pleuroceridae) accounted for 70 to nearly 100% of the Mollusca 
totals (by weight). Below Lagrange the Viviparidae (and Pleuroceri- 
dae) were largely replaced by Sphaeriidae in all zones, the weight per- 
centages of that group rising to a range between 84 and 100%. 
Bortom Fauna, Inirnots River, 1915 
PERCENTAGES OF AVERAGE TOTAL VALUATIONS BY WEIGHT 
CONTRIBUTED By MoLLuscA 
4—7-ft. | 1—3-ft. 
Channel zone zone 
Chillicothe to Copperas Creek dam 96.7 ite 92.1 
Copperas Creek dam to Havana 99.5 98.8 98.8 
Havana to Lagrange 86.3 98.1 98.9 a 
Lagrange to Grafton 31.6 65.8 35.8 
Bottom Fauna, ILLInois River, 1915 - 
Composition oF Motitusca ToTats. (PERCENTAGES BY WEIGHT*) 
Viviparidae and Sphaeriidae and small 
Pleuroceridae Gastropoda 
scar 4—7-4t. | 1—3-£t 4—1-£t.| 1—3-ft 
Channel zone zone "| Channel zone ; zone : 
Chillicothe to Copperas 
(Gucryie Glaira 78.5 78.1 70.7 25 21.9 29.3 
Copperas Creek dam 7 9 
TURETA Tae 100.0 22.5 97.7 trace eres) 2.3 
Havana to Lagrange 84.3 84.8 89.7 15.7 15.2 10.3 
1 i 
Lagrange to Grafton 15.8 none 13.2 84.2 100.0 86.8 
* Pound values on which these percentages are based are shown in following 
tables. 
