100 ILLINOIS BIOLOGICAL MONOGRAPHS [198 
EXPLANATION OF PLATE 
Fic. 3. Old stream bed of Salt Fork in Busey woods, north of Crystal Lake, Urbana. 
Original habitat for Planorbis pseudotrivolvis. Station 3. 
Fic. 4. Junction of Middle Fork with Salt Fork to form the Big Vermilion River. Look- 
ing south from road bridge crossing Middle Fork. September 26, 1920. 
Fig. 5. Middle Fork a fourth of a mile above junction with Salt Fork. Bed of river 
exposed in foreground. Station 29. September 26, 1920. 
