518 
much greater proportion of limnetic individuals, with a cor- 
responding decrease in the relative numbers of the littoral in- 
dividuals. This is very apparent throughout the whole season 
covered by the collections. Thus in autumn months, when 
Melosira, Synura, the ciliates, Syncheta, and various Brachi- 
onide characterize the potamoplankton, we find them but 
sparingly represented, or, as in the case of Syncheta and the 
ciliates, wholly absent from the creek plankton. So, also, in the 
winter months Brachionus dorcas and Cyclops bicuspidatus, so 
characteristic of the river plankton, form no part of the popula- 
tion of the creek. The perennial and abundant Polyartha 
playtyptera was at no time found in thecreek. That limnetic 
species are not wholly absent from the creek is shown by the 
presence of Melosira, Synura, Notholca jugosa, Kudorina, and 
Trachelomonas, but their occurrences are isolated and their 
numbers few. The creek is thus not a center of distribution 
for such planktonts as these, their presence and numbers in 
the river being practically independent of their appearance in 
the tributary. 
The abundance of diatoms in the creek waters suggests 
that these may find centers of distribution here. Most of the 
species, however, are quite as abundant in the river, with the 
exception of the Swrirellas. These are present in small num- 
bers in the potamoplankton, and are often moribund. 
3. The creek waters act as diluents of the potamoplank- 
ton. The character of the Quiver Creek plankton and the 
quantitative studies on Quiver Lake support this view. 
The contribution of Quiver Creek to the potamoplankton 
is thus largely of littoral species and of small quantity, and its 
effect is that of a diluent of the potamoplankton. 
The following table, which gives the relative number of 
species in the plankton of Illinois and Spoon rivers and Quiver 
Creek, demonstrates the small number of species found in the 
creek, and the monotony of its composition as shown in the pre- 
dominance of the Rhizopoda and the diatoms. The relative pau- 
city of the Mastigophora in its fauna may be due in part to the 
