163] FAUNA OF BIG VERMILION RIVER—BAKER 65 
have been clearly pointed out in the paper referred to above. Two speci- 
mens, one living, referable to this species were found in Salt Fork about 
two miles below St. Joseph, associated with typical ¢rivolvis. 
60. Planorbis (Helisoma) antrosus Conrad. 
A single, small, bleached shell of this species was found near the cement 
bridge northeast of Sidney. It may have been washed from a Pleistocene 
fossil deposit near by. None were collected living. 
61. Planorbis (Gyraulua) parvus Say. 
This small species was found in limited number associated with Planor- 
bis trivolvis and Galba parva in the low ground subject to spring overflow 
south of the interurban bridge at St. Joseph. It was typical in form as 
compared with authentic specimens collected near Philadelphia by Dr. 
H. A. Pilsbry. None were found in Salt Fork or in the Sangamon River. 
FAMILY LYMNAEIDAE 
62. Galba parva (Lea). 
This tiny pond snail was found at but one place in the Salt Fork. A 
dead shell was obtained at the iron bridge, a mile north of Sidney. In 
the low ground south of the interurban bridge at St. Joseph before referred 
to, parva occurs abundantly in the bed of a small stream which has water 
in it only in spring and early summer. On the Sangamon River, this species 
was noted in abundance on wet mud flats bordering the margin of the 
stream. This locality was about three-fourths of a mile below Mahomet. 
The polluted water at St. Joseph appears to have little effect on this species 
or the other pulmoniferous mollusks associated with it. It is probable 
that at the times of high water so much oxygen is mixed with the upper 
layer which overflows these low places that the ill effect of sewage pollution, 
from decomposition of organic matter, is so reduced in quantity and quality 
as to be little noticed by these air-breathers. 
63. Galba humilis modicella (Say). 
This is usually a very common species where it occurs at all. It is rare, 
however, in the Big Vermilion, scattered specimens, mostly dead shells, 
being found at four stations. It was collected living in the drainage ditch 
above Urbana, in Crystal Lake (Zetek), and a large typical individual was 
found alive on the mud bordering the stream a short distance below the 
mouth of Spoon River in Salt Fork west of St. Joseph. The species occurs 
sparingly in the Sangamon River associated with Galba parva at the locality 
mentioned under that species. In a small pond in the gravel pit north of 
the interurban tracks west of the Middle Fork, modicella occurs living 
among cat-tails (Typha) associated with Physa gyrina. 
( 
