54 W. C. ALLEE. 



above were taken from the County Line Creek, which flows into 

 Lake Michigan near Glencoe, Illinois. About three sixteenths 

 of a mile above its mouth the creek branches and its total length, 

 including tributaries, is about half a mile. This is a physio- 

 graphically young stream in the intermittent rapids, permanent 

 pool stage with numerous stones scattered over its bed. It is 

 ranked by Shelford ('13) as the third in age of his series of twenty- 

 six stations in the study of stream ecology. 



Stream isopods also occur in the following streams: 



1. Pettibone Creek, a large stream north of the County Line 

 Creek. Shelford ranks this as next older than the County Line 

 Creek in his physiographic series. In many ways it resembles 

 the younger stream but is more permanent and contains less 

 sewage. The isopods are much less abundant. 



2. North Branch of the Chicago River at Des Plaines, Illinois. 

 Shelford ranks this as a "moderately swift medium-sized stream" 

 and as station 16 in his stream series of 26 stations. When visited 

 in August, 1911, the stream was reduced to a series of pools 

 without connection, hence the part visited was younger than 

 Thorn Creek. The isopods were found among the numerous 

 stones on the bottom of the pools. 



3. Thorn Creek at Thornton, Illinois. Shelford lists this as 

 station 12 in his stream series. This is a rapid permanent stream 

 about 20 feet wide at ordinary flow. There is much sewage 

 present from Chicago Heights. The isopods here are scarce, 

 being confined to (a) protruding roots under overhanging banks; 

 (b) under the edge of rocks in the wide shallow riffles; (c} in 

 embayments or among relatively permanent catches of drift. 

 This stream represents about the largest, oldest stream in which 

 the stream mores of these isopods can maintain themselves. 

 Isopods may occur in very old streams of the sluggish base-level 

 type, as in the Fox River at Cary, Illinois, but the part of the 

 stream in which they are found is ecologically equivalent to a 

 mature pond (Station 35, p. 34; Shelford '13). 



From this study of the streams which contain isopods it was 

 found that the stream mores of Aselhis communis are most 

 numerous in young streams of the intermittent rapids, permanent 

 pool type; especially those that contain some sewage, and that 



