354 



THE AMERICAN NATURALIST 



[Vol. XLI 



■(I) 



c. g., 



at the 7.25 mile mark. (Enters 

 from the left side.) 

 at the 7.5 mile mark. 



Takiiif? the common bullhead, Amemrus nebulosus, as an 

 exam})le, one is able to read from the chart, concerning its distri- 



of the stream, gradually decreasing in abundance at the end of the 

 second mile. Throughout the middle course where rock or 

 gravel bottoms and swift water or rapids occur this species is 

 absent. In the upper course where the current and bottom are 

 influenced by the dam, located 5.5 miles from the mouth, it re- 

 appears. In the latter iiistMiicc, its |)i'('s(Mice so near the head- 

 waters is due to canal ovci liows at the 7.5 mile point. It seldom 

 frequents water less than 4 feet (ieej). In both ranges, the drop 

 in the stream is slight, so that the current is just perceptible at 

 the mouth, and imperceptible at the dam. At these two points 

 the stream's width is respectively 27 and 32 feet. In both places 

 a muddy bottom obtains. In the lower course the stream lies 

 on a delta formation while in the upper course the underlying 

 stratum is glacial drift. 



The Johnny darter, Boleosoma nigrum, occurs at the source 

 of the creek, due to a contribution at floodtime from another 

 stream across the divide, the two sources being on the same level 

 and continuous at some seasons. ^I'he falls on either side of the 

 4 imMc ]»()iiit would pircludc its reaching the source from the mouth. 



deductions might be drawn. Should it seem desirable to make 

 the work more intensive, to restrict it to a limited portion of a 



bution, etc., the following: 



