62 ILLINOIS BIOLOGICAL MONOGRAPHS [160 



The rujum 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 integrum ohesum 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. 

 " 33 " 22 " " 19 " 13 mm. 



" 30 " 21 " " 18 " 12 mm. 



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 environmicnt. There are many normal and favor- 

 able habitats for this moUusk 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 on a 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 subsoUdum 

 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 



