BROOK, CREEK, AND RIVER 279 



thoroughness. We must be content, therefore, to indicate 

 briefly changes in animal and plant population as the stream 

 grows in size from brook to creek and to river. Then attention 

 may be called to the societies of rapid water as contrasted with 

 those of sluggish water, and finally we may take up very briefly 

 the societies of intermittent streams, of spring-fed streams, and 

 of acid streams. 



Take first the distribution of some of the river clams in the 

 Illinois River and its branches. The data are taken largely 

 from Baker's " Catalogue of the Mollusca of Illinois," Bulletin 

 of the Illinois State Laboratory of Natural History, Volume VII, 

 Article VI, and his " Mollusca of the Chicago Area, " Part I, "The 

 Pelecypoda," Publication of the Chicago Academy of Sciences. 

 The larger branches of the Illinois from its mouth toward its 

 source are in order, (i) the Sangamon, (2) the Spoon, (3) the 

 Mackinaw, (4) the Vermillion, (5) the Fox, (6) the Kankakee, 

 (7) the Desplaines. It is the union of the last two that makes 

 the Illinois. The more important branches of the Desplaines 

 are (8) the DuPage, (9) Hickory Creek, (10) Salt Creek. The 

 smaller creeks or runs are mere brooks and do not support a 

 clam population. Of the genus Lampsilis only two get up into 

 Salt Creek, .namely, L. luteolus and L. ellipsiformis. Four more 

 are found in Hickory Creek, L. ventricosa, L. fallaciosa, L. iris, 

 and L. parva. All of the foregoing except one, L. fallaciosa, are 

 reported from DuPage River, and in addition four others, L. liga- 

 mentina, L. anadontoides , L. recta, and L. alata. All of the fore- 

 going are found in the Illinois and its larger branches, and in 

 addition six other species that do not get up into the Desplaines. 



Anadonta marginata is found in Hickory Creek but not in 

 Salt Creek. A. grandis and A. grandis footiana are in the 

 DuPage but not farther north. A . imbecilis is in the Desplaines 

 but not in its branches. The Quadrulas are almost entirely 

 confined to the larger streams. Q. rubiginosa is reported from 

 Salt Creek and Q. coccinea from the DuPage. There are eighteen 

 other species in the Illinois and its larger branches named above. 



