: 535 
be excluded from the averages. The ratio in 1898 is 1 to 56, 
and as this isa year of normal hydrograph it may represent 
more nearly the true proportion in the production of rotifers 
in the two streams. The data in the remainder of 1897, in 
1896, and in 1899, so far as they go, sustain this higher ratio. 
This ratio is somewhat greater than that of the alge, rhizopods, 
and infusorians, which form earlier links in the chain of food 
relations. The rotifers, on the other hand, depend upon these 
groups for food, and become later links in the chain, and conse- 
quently do not, for lack of time, attain the development in 
tributary waters that they doin the main stream. The diatoms 
and Mastigophora also form considerable elements in the food 
of the rotifers, and since these groups are proportionally less 
frequent in Spoon River than in the main stream, the deficiency 
in this food element may be one cause of the lesser develop- 
ment of rotifers in the tributary. Most rotifers, however, do 
not exhibit such limitations in diet, and the effect of this de- 
ficiency in food must be largely quantitative. 
The number of species of rotifers found in Spoon River is 
much less than that in the main stream (44 to 107—see p. 519), 
and none peculiar to the tributary was noted. The Bdelloida, 
which are principally shore-loving and bottom forms, constitute 
a relatively greater proportion of the species and individuals in 
the tributary, though their absolute numbers per cubic meter 
of water are rarely in excess. 
In the low-water period of 1897 the limnetic species com- 
mon in the main stream and in the residual backwaters reached 
an unusual development in Spoon River, even in excess of that 
in the main stream. The conditions then prevalent, the higher 
temperatures of a late autumn, stagnating water with little or 
no current, and the absence of the usual autumnal flushings, 
combined to favor the unusual phenomenon. During this sea- 
son the contributions of Spoon River to the rotiferan fauna of 
the main stream would increase the amount of rotiferan plank- 
ton therein, though not diversifying it—as will be seen on com- 
parison of the relative number of species in the two streams at 
that season. 
