293 



both species have heen founci most frequently in the eastern 

 part of the state. A close comparison of the distribution maps 

 shows that both have been taken from eighteen of the thirty /o- 

 culitics in which the less abundant one was found; and they have 

 been taken together in seventeen of the one hundred and five 

 ''o//erf/o«s containing either or both. 



A comparison of their local i>references indicates a clo.se 

 agreement in ecological relationship. Each of the species was 

 found in the larger rivers in 8 per cent, of the collections; zonule 

 in 97 per cent, of those from the smaller rivers and creeks, and 

 ccenileKtn in S9 per cent. — the remainder of the latter coming 

 from lowland lakes and ponds ( 1 per cent, i and from various 

 miscellaneous .sources. Eighty-nine per cent, of the collections 

 of zonale and 83 per cent, of those of ((eruleuin were f rom .slreams 

 of swift or moderate flow: S9 per cent, of zonale and 92 percent, 

 of caridetiiu. from rock and sandy bottom. The only notable 

 difference between these species is the preponderant disposi- 

 tion of zonale towards the smaller rivers ra* ' the 

 streams classed as creeks. 



The next highest coefficient i5.69» is that of HadiopUrus 

 phojrorep/ialus and Etlieo-^ioma zonale (1418 -'Ij? which 



have occurred together sixteen time- ' adred and 



one collections of one or the other. „ :.. _-.. een taken 

 from seventeen of the thirty localities in which we have found 

 zonale. The general distribution of the two differ but little, 

 except that zonale is very : --s abundant thsLU phoxo'^-ep ft a - 



/«.<. and has been limited : . jre closely to the Illinois and 



Rock river basins. The ecological ratios for zonale and phoxo- 

 ■ ''ilus respectively are. — lai-ger rivers. 3 per cenL and 7 p>er 

 cent.: smaller rivers. 74 per cent, and 56 per cent.: creeks. 23 

 per cent, and 26 per cent. : lakes and ponds. U and 3 per cent. 

 The ratios of preference for rapid and slow waters respectively 

 are still more closely approximate — S9 per cent, of zonale and 

 57 per cent, of phox-i^^plialm from moieraie or rapid cuxrents. 

 The preferences of the two species for rock and sandy bottom 

 are similarly close — 89 per cent, for z:<nah and 94 per cent, for 

 y^i .:■■:■ phalus. 



