291 



pondevant abundance of the tirst in the north half of the state 

 and of the second in central and southern Illinois. Among the 

 twenty-nine localities from which the first of these species was 

 taken, aiul the tifty-four for the second, there were hut two in 

 which both were found, and at each of these localities they oc- 

 curred in only one collection. That is, in one hundred and 

 eighty-eight separate collections of one or the other of these 

 species from these various localities, the two were taken together 

 but twice — a fact to be connected partly with the limitation of 

 EfhfOf<fo)iin zonale to the northern half of the state, and partly 

 with differences in the bodies of water in which these species 

 habitually occur. Twenty-one per cent, of our collections of 

 Jessiw came from the larger rivers, and only 3 per cent, of 

 those of zi>n<(le\ 19 per cent, of jessht', from the smaller rivers 

 and creeks, and 74 per cent, of zonule-, 24 per cent, of Jessice, 

 from lakes and ponds, and none of zoiui/c. 



BdhnsoiiKi (■(iiiiHnini and Eflieosfoii/a zaiuth- [ 144S and 1461), 

 whose coefficient of association is but .39, furnish an e.xample 

 of the relation of distribution already referred to, the area of 

 the two species overlapping, but not coinciding throughout — 

 that of zoiuih expanding to the northward and that of (■(innirtini 

 to the southward. Partly in consequence of this fact, we have 

 but a single joint occurrence of these species out of one hun- 

 dred and thirty-eight collections containing one or the other. 

 Their ecological relations, as shown by Table VI., are also quite 

 unlike. Bolrosviiia c(uiinrt(iii occurring in sluggish or stagnant 

 waters five times as frequently as the other species, and in 

 waters with a muddy bottom in a still greater differential ratio. 

 The low associative coefficient (.63) of HadropfcrHs phoxo- 

 cepli a I II s -dnd Holrirlif/iifs fiisi/dniiis (141S and 1494) is largely 

 explained by the difference in preponderant distribution, the 

 former being commonest in the Illinois valley and to the 

 northward generally, while the latter is much the most abund- 

 ant in the Wabash system and in extreme southern Illinois. 

 In one hundred and thirty-eight collections containing one or 

 the other of these species, they have occurred together but 

 three times,— twice in branches of the Little Wal)ash River and 



