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 Urbatia, 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 sayii Tappan, 

 from Urbana (BakeT, 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 trivolvis 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 trivolvis being rare. The differences between this species and trivolvis 



