62 ILLINOIS BIOLOGICAL MONOGRAPHS [160 
The rufum from Homer Park have the spire whorls more or less gibbous, 
strongly shouldered, the first three whorls seeming to be telescoped into 
the later whorls. In this respect they resemble imfegrum obesum as sug- 
gested above. Measurements of a few of the Homer Park specimens are 
given below (Z11168): 
Length, 37; breadth, 24; aperture length, 21; breadth, 14 mm. 
SEG ABST EMD DT a hon BEN AO) UN it 3 Sree 
GS SOS, Ct OI peetel ene Bee <0 ae eerie 
To this and other lots of Campelomae from the Big Vermilion River the 
statement of Lewis may be well applied: “‘These and many other forms in 
my collection, all part of a series, go far to show that it is unsafe to attempt 
to decide the limits of species from a few individuals” (1875:337). 
The distribution of this species in Salt Fork is interesting and sug- 
gestive. Living specimens, small and few in number, were found over two 
miles upstream in Spoon River. Dead, mostly old and bleached shells, 
were collected at nearly all stations in the Salt Fork, but living shells of 
rufum were not seen above the station two miles north of Sidney. Here 
only one living specimen could be found. A mile farther down the stream 
another living specimen was collected. The presence of so many dead 
shells with so few living individuals above the Homer Park dam indicates 
clearly an unfavorable environment. There are many normal and favor- 
. able habitats for this mollusk in this stretch of nearly twenty miles in Salt 
Fork and the unfavorable agencies must be wholly those contributed by 
Man—the disposal of sewage and other wastes by means of this stream. 
Below Homer Park dam the species is abundant and as fine as can be 
found anywhere. Rufum is rare ona sand and gravel bottom and abundant 
on a mud bottom. 
The Campelomae from the Sangamon River at Mahomet are also refer- 
able to rufum. The spire is longer and the shell narrower, however, than in 
the Salt Fork specimens, and there is no tendency to vary toward the 
obesum form of shell. The interior of the aperture is slightly pinkish. One 
specimen from Mahomet has a very heavy shell recalling the swbsolidum 
of Anthony, a common species in most parts of Illinois but absent from 
either of the rivers under consideration. Reversed individuals are rare, 
only one specimen being found in the Sangamon River, a mile below 
Mahomet. This is a young individual. 
The air-breathing snails, belonging to the genera Physa, Ferrissia, 
Planorbis, and Galba, are better able to withstand the ill effects of sew- 
age and other stream pollution than are their relatives, the snails and 
clams that take their oxygen directly from the water (dissolved oxygen). 
They were therefore found in Salt Fork in places where the water breathers 
were entirely wanting, as at St. Joseph and the first stations below. It has 
