SANDY BOTTOMED STREAMS IOI 
occasional aquatic insects, and hair-worms (Gordius). In some localities 
bivalved mollusks (Sphaeridae) and leeches are numerous. 
Under primeval conditions beavers are associated with the pool for- 
mation. They build dams which contribute to the deepening of the 
water of the pools. For a good account of their habits see citation gg). 
An old beaver dam is supposed to have turned the waters of the 
DesPlaines out of the Chicago River and down the Chicago outlet. 
Characters of the formation: The mores of the pool formation are dis- 
tinctly those of partially burying the body just beneath the surface of 
the fine gravel and moving against the current. The few animals that 
make cases usually use gravel or sand-grains. A single caddis-worm 
makes its case from small sticks such as commonly lodge in eddies. 
Some of the fishes breeding in these situations cover their eggs (50). 
Some fishes orient the body and swim upstream as a result of seeing the 
bottom apparently move forward below as the fish floats down (94). 
They behave the same if put into a trough with a glass bottom and the 
trough drawn forward. Some orient also when their bodies rub against 
the bottom when floating downstream. 
5. THE COMMUNITIES OF SANDY BOTTOMED STREAMS (SHIFTING BOTTOM 
SUB-FORMATIONS) 
(Stations 22-26; Table XXIV) 
We have studied the upper course of the Black River, the upper 
course of the Calumet River, and the Deep River, and two or three 
tributaries of Lake Michigan near South Haven. The kind of material 
eroded is of the greatest importance in determining the mores present in 
a stream. The streams of the eastern part of our area are in till which 
is sandy and their bottoms are sandy. This material is always slipping 
and moving downstream. There are few large stones. The bottom is 
not suitable for animals. The swift-water animals are almost entirely 
absent. The forms present are those which belong to moderately swift 
water. 
Composition and subdivisions.—Such streams are poorly populated. 
Their mores resemble those of the formations of the pools of streams 
eroding coarse material, but the shifting is so much more general and the 
species found so different, that it has been thought wise to separate the 
two. In the Michigan streams there are in summer a few scattered 
plants, which support a considerable number of insects; some of the 
brook beetles (Parnidae) are found attached to them. The logs and 
roots that happen to be in the water are important; they are the only 
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