286 



sis of their associative ratios, I have distinguished as typical 

 and the six less typical species, shows that the more typical 

 group occurs in the smaller rivers and creeks in 8S per cent, 

 of these collections, and the less typical in 47 per cent.; the first 

 group, in swift waters in 88 per cent, of the cases, and the sec- 

 ond in 62 per cent.; the hrst, in rocky or sandy streams in 91 

 per cent., and the second in 66 per cent. That is, the frequency 

 of occurrence of the less typical species in small rivers and 

 creeks is 53 per cent, of that of the more typical species; in 

 swift waters it is 71 per cent., and on I'ock and sandy bottoms 

 it is 72 per cent., — ^an average of 65 per cent, for these three 

 factors. These purely ecological ratios agree in a significant 

 manner with the corresponding averages to be drawn from the 

 tables of associative frequencies, as may be seen by reference 

 to Table I. If we average separately the totals for the first six 

 and the last six species of that table, we find the average of the 

 latter group to be 63 per cent, of that of the former — the dif- 

 ference in degree of associative affiliation is essentially the 

 same as the difference of ecological relationship, the one con- 

 clusion confirming, and likewise explaining, the other. 



It is further to be noticed, of the ecological affinity of the 

 six selected species, that no one of them has been found in up- 

 land or glacial lakes; that their occurrence in lowland lakes, 

 ponds, and sloughs — an average of only 1 per cent. — is so rare 

 as to be negligible; and that, omitting Anunocnjpta pelhfcida, 

 which is in some respects peculiar, the frequency ratio for the 

 larger rivers ranges from 3 to 9 per cent., with an average of 

 only 6.5 per cent, for these species. This uniformity of their 

 ecological relationships, which makes of them a well defined 

 ecological group, is the explanation, of course, of their high de- 

 gree of associative affiliation. The most notable specific differ- 

 ences among them are the relative frequency of A»nnocri/pfa 

 pelliicidii, and the absence of Diplesion hJciniioides, in my two 

 hundred and ninety-three collections from the larger rivers. 



The six less typical species, on the other hand, have little 

 in common except their difference from this more typical 

 group. Boleosoma nigrum, of which we have two hundred and 



