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siderable in comparison with that from the other sources. 
Wherever the river encroaches upon the bluffs, as, for instance, 
below Havana, seepage areas of some extent and springs of 
some size are to be found. Such banks, however, in immediate 
contact with the main stream itself, are of very limited extent. 
Furthermore, their contributions to the plankton are relatively 
still smaller. These springy banks abound in life—planarians, 
amphipods, isopods, oligochztes, and rhizopods, mainly limic- 
olous species, which rarely leave their habitat to enter the 
river with the spring water. Such springy banks, exposed to 
midsummer’s heat during low water, do at times teem with 
species common in the plankton. For example, a bank of this 
sort on the levee at Havana was covered with a brownish scum 
composed principally of Synedra acus, a diatom abundant in 
the plankton at cooler seasons of the year. The temperature 
of spring waters along the bluffs in midsummer is about 60°. 
Elsewhere in the warmer waters of this springy shore are to 
be found patches of green and red scum, where Euglena viridis 
and HL. sanguinea were abundant, both species being common in 
the plankton at that time. Tiny rills of cool water traverse 
the oozy bank and carry stray individuals of these various 
species into the river, but their clear waters are poor in com- 
parison with the brown water of the stream which they join, 
turbid with plankton. Their contributions are thus insignifi- 
cant in amount and, while adding a trifle to its diversity, their 
main action is that of diluents of the potamoplankton. 
RELATION OF TRIBUTARY STREAMS TO CHANNEL PLANKTON, 
The relation of tributary streams to the potamoplankton in 
the channel of the Illinois is a much simpler problem than that 
presented by the backwaters. Their contributions enter the 
river in well-defined channels, and the areas of their respective 
basins are an index to the quantity of water they bring to the 
river. Their share in the formation of the potamoplankton can 
thus be more readily tested and estimated. Under conditions 
prior to the opening of the Chicago drainage canal the river 
