100 ILUNOIS BIOLOGICAL MONOGRAPHS [198 



EXPLANATION OF PLATE 



Fig. 3. Old stream bed of Salt Fork in Busey woods, north of Crystal Lake, Urbana. 

 Original habitat for Planorbis pseudotrivolvis . Station 3. 



Fig. 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. 



