64 ILLINOIS BIOLOGICAL MONOGRAPHS {162 
FAMILY PHYSIDAE 
56. Physa gyrina Say. Tadpole Snail. 
This snail is usually abundant wherever found. In the old cut-offs 
of the Salt Fork above Urbana, as well as in the ditch north of Urbana, 
it is abundant and quite typical with long, slender shell and spire, the 
immature individuals with a short, dome-shaped spire. Below St. Joseph, 
where it occurs sparingly, the shell is broader and even in adult specimens 
the spire is more or less dome-shaped. Gyrina is more common above than 
below the dam at Homer Park. The species is more characteristic of slow- 
moving, pond-like bodies of water than of larger streams. It occurs also 
in Stony Creek near Muncie, in a small pond near Middle Fork, and in 
the Big Vermilion below Middle Fork. 
57. Physa crandalli Baker. 
Specimens of a Physa with a shouldered whorls, a wide body whorl, 
the shell thick and heavy for the genus are referred to crandalli. This 
mollusk is abundant in the drainage ditch above Urbana associated with 
gyrina. None were found in the cut offs of the old stream and the species 
probably does not inhabit the pond-like habitats in which gyrina is usually 
found, preferring running water. The specimens referred to sayzz Tappan, 
from Urbana (Baker, 1906:99) are also this species. It is noteworthy that 
living specimens of this species were collected at St. Joseph where polluted 
conditions are bad. None were found below the iron bridge one mile 
north of Sidney, or in any part of the Salt Fork below this point. Charac- 
teristic specimens were collected in the Big Vermilion below Middle Fork, 
on a stony bottom in riffles. 
FAMILY PLANORBIDAE 
58. Planorbis (Helisoma) trivolvis Say. Wheel Snail. 
This species of wheel snail is apparently not common in Salt Fork, only 
scattering specimens being found along the stream. It occurred more 
abundantly in a small stream, dry in summer, which runs through low. 
swampy ground on the east bank of Salt Fork south of the interurban 
bridge at St. Joseph. Living érivolvis were not found in the stream above 
the first bridge below St. Joseph, twelve miles below Urbana. Even this 
air-breathing snail seems to be unable to live in any abundance in the 
polluted water of South Fork. 
59. Planorbis (Helisoma) pseudotrivolvis Baker. 
This recently described wheel snail (Baker, 1920:123) occurs abun- 
dantly in the old stream bed (cut-offs) of the Salt Fork near the Woodlawn 
cemetery, Urbana, and it is here the predominating species of the genus, 
true ¢rivolvis being rare. The differences between this species and ¢rivolvis 
