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, Si/nrJia4a, and various Bravlii- 

 onidce characterize the potamoplankton, we find them but 

 sparingly represented, or, as in the case of Syncluda and the 

 ciliates, wholly absent from the creek plankton. So, also, in the 

 winter months Brachionus dorcas and Cyclops hicuspidatus, so 

 characteristic of the river plankton, form no part of the popula- 

 tion of the creek. The perennial and abundant PoJyartha 

 playtyptera was at no time found in the creek. That limnetic 

 species are not wholly absent from the creek is shown by the 

 presence of Melosira, Synura, Nofholca jugosa, Eudorina, 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 Surire/las. 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 Mastigopliora in its fauna may be due in part to the 



