287 



thirty-six collections, is an abundant and wide-ranging species, 

 with comparatively feeble ecological preferences, as is shown 

 by the fact that 15 per cent, of these collections are from lakes, 

 32 per cent, from still waters, and 11 per cent, from those with 

 a muddy bottom. Petri >ta rojjrudes (si.xty collections) makes a 

 similar showing, this being also a lake species in part (19 per 

 cent.); but it differs from the preceding in the fact that it has 

 occurred more frequently in the larger streams (10 per cent.), 

 less frequently in still waters ( 7 per cent.), and not at all on 

 muddy bottoms. Cottocjaster slnnnanli, so far as may be judged 

 from our sixteen lots of this species, is peculiar in its frequency 

 in the larger i"ivers (55 per cent.) and the lowland lakes (18 

 per cent.), and in its avoidance of the smaller streams (only 4 

 per cent, in the creeks and smaller rivers). Etheosfoma Jessiie 

 (one hundred and fifty-eight collections) is an indifferent 

 species, and occurs in almost equal ratios in large rivers, small 

 rivers, creeks, and lowland lakes. Boleichthys fusiformis, which 

 we have taken fifty-six times, is rare in the larger rivers, and 

 seems to be the commonest of all our species in the upland lakes. 

 Boleusoina raiininnii (one hundred and seven collections) is 

 somewhat less indiscriminate in its local prefei-ences. It is com- 

 monest in creeks (42 per cent. ) and relatively rare in the larger 

 rivers (9 per cent.). It apparently has no marked preference 

 for swift waters over slow, nor for a hard bottom as compared 

 with one of mud. 



The ecological heterogeneity of these least typical species is 

 reflected in their relatively feeble associative affiliations, these 

 six species having a mutual associative ratio (derived from 

 Table II.) of 1.4, while the corresponding ratio of the first six 

 more typical species of Table II. is 3.28. 



Association and Distribution. 



The association of species may lie looked upon as a conse- 

 quence of their distribution. Species of wholly different gen- 

 eral, or geographical, distribution can, of course, never be asso- 

 ciated; and the same is true of those of wholly unlike local dis- 

 tribution. Those whose areas of general distribution merely 



