24 ILUNOIS BIOLOGICAL MONOGRAPHS [122 



GENERAL DISTRIBUTION OF THE MOLLUSK FAUNA 



It will be noted in Table III that the mussels were found in greatest 

 abundance in a bottom composed of sand or gravel, or both, and were 

 fewer in number of species, as well as in individuals, on a mud bottom. 

 As a rule the. mussels were found in abundance on the shallow riflOles and 

 were often absent from the deeper places in which the bottom was composed 

 of soft mud. A notable exception to this rule occurs at Homer Park where 

 the largest mussel fauna is found in a mud bottom, and where this kind of a 

 bottom produced eleven more species (28) than did the sand and gravel 

 riflBes a short distance below (17). This station has been used by Professor 

 Frank Smith for many years as a field habitat for his zoology classes and 

 the mussel fauna is, therefore, better known than that of any other locality 

 on the stream. Although visited several times a year for nearly a score of 

 years, there seems no diminution of the fauna in either species or individ- 

 uals. A day spent at this station, during which two collectors examined the 

 stream, yielded 24 of the 28 species. This indicates the great abundance of 

 the fauna, which may be due in large measure to the aerating influence 

 of the dam situated just above the collecting grounds. 



The stream below the dam at Homer Park is an excellent place in which 

 to study the ecological conditions governing the distribution of the mollusk 

 fauna in a small stream. There is first a very shallow stream below the 

 dam (Fig. 14) flowing over gravel and boulders, in which a few gastropods 

 and small bivalves (Sphaerium) live in considerable abundance (Fig. 15). 

 Mussels are rare. Then follows a comparatively deep area of the stream 

 (2-3 feet) flowing over a bed of fine sand or mud in which mussels are 

 abundant and gastropods rare (Fig. 13). This is followed by a moderately 

 shallow stretch of the stream (1-2 feet) which flows over a bottom of coarse 

 sand and gravel in which mussels are found in abundance, ahhough not as 

 numerous in species, as in the mud habitat. A few gastropods live here. 

 These conditions are exceptional in Salt Fork and are not duplicated in 

 any other part of the stream above Danville. 



Some species of mussels, as Anodonta, Anodontoides, and Uniomerus, 

 prefer a mud bottom and thrive only in such situations, but the great 

 majority of the naiades prefer a sand or gravel bottom in water with 

 considerable current, and this is the reason they are so abundant in the 

 riffles of all streams. Among the smaller bivalves (Sphaerium and Pisi- 

 dium), the majority of species prefer a mud or fine sand habitat. Of the 

 snails or gastropods, Pleurocera and Goniobasis are usually found in a 



