SLUGGISH STREAMS 103 
community is distinctly of the pond type in its general mores. Stagna- 
tion and low oxygen content and the partial drying of the stream are 
tolerated by all the residents. 
b) Sluggish river formations.—The conditions in sluggish rivers are 
different from those in smaller swift streams in many respects. The 
bottom is for the most part of fine materials; there are no rocks. The 
difference between pools and rapids no longer exists. The river is a 
gently flowing mass with relatively little distinction as to different parts. 
The margins of such streams are lined in summer with typical rooted 
and holdfast aquatic plants. The small bays and out-of-the-way spots, 
out of the current, support bulrushes and sometimes cattails. We can 
distinguish several formations in the Fox River: (1) The pelagic forma- 
tion, (2) the formation of sand and silt bottom (association of sandy 
bottom where the current drags in midstream or beats against the 
shore; association of silt bottom where least current is present), and 
(3) the formation of the zone of vegetation. 
Pelagic formation: This is well developed in the larger rivers, e.g., 
the Illinois River (77). While the Illinois no doubt differs from the Fox 
in many respects, doubtless the general features are much the same. 
It does not differ greatly from that of Lake Michigan. 
Burrowing May-fly or sand and silt bottom formations: On the 
bottom in ten feet of water we have found mussels (Anodonta grandis 
and Quadrula undulata), the snail (Goniobasis livescens), bloodworms 
(Chironomidae), green midge larvae (Chironomidae). On the old mussel 
shells were large colonies of the bryozoan Plumatella and occasional 
caddis-worms (Hydropsyche) (Figs. 39, 40, p. 96). On sandy bottom, 
conditions near the margin are similar to those on the bottom. We 
find here also an occasional snail (Goniobasis, Pleurocera, and Campe- 
Joma), the midge larvae and bloodworms, occasional burrowing May- 
fly nymphs, and a number of mussels (Unio gibbosus and Quadrula 
rubiginosa being the most characteristic). There is also an occasional 
specimen of the long-legged dragon-fly nymph (Macromia taeniolata) and 
the black-sided darter. A considerable number of these species occur 
in the stillest pools of Hickory Creek, indicating the types that will 
dominate later. Silt is often found in particular spots. The most 
characteristic animals in this are the large mussel (Quadrula undulata), 
the burrowing May-fly nymph (Hexagenia sp.), and the bloodworms 
(Chironomidae). There are also the worms (Annelida) which burrow in 
the mud and protrude their anterior ends, often also the common 
mussel (Lampsilis luteola), the Sphaeridae, and the mud leech (Haemopis 
