ANIMAL ECOLOGY OF JOHNSON COUNTY © 33 
it can support are many except, of course, such animals as would 
require turbulent water for oxygenation. In these mature 
streams there is an abundance of organic matter, a correspond- 
ing wealth of micro-organisms, and with this a great variety of 
small crustaceans and insect forms. These in turn feed other 
larger forms which in their numbers provide abundant food for 
a large variety of vertebrates, chief among which are the fishes. 
In such a stream as the Iowa River these fishes often attain con- 
siderable size, carp and catfish taken in the vicinity of lowa City 
being reported to have weighed more than thirty pounds. 
Necturus, the lamphreys, and the gars are forms that are fre- 
quently met with in addition to the regular edible fishes. 
Within the river itself there are mary sub-habitats, each with 
its own peculiar association of animal life. The quantity of 
water provides for an extensive free-swimming animal associa- 
tion. There is a bottom association which varies with the vari- 
ations in character of the bottom from those forms adapted to 
live in soft oozy mud to those capable of supporting themselves 
on gravely rocky bottom. To these must be added a shallow- 
water association, or those forms usually distributed in the nar- 
row strip of shallow water at the very edge of the stream. Then 
there is the surface-film association, containing such forms as are 
adapted to existence on the surface film, such as the water- 
strider. The rwer bank habitat is practically identical with the 
ereek bank habitat, and for that reason is discussed here only. 
It consists of such forms as require burrows for their homes but 
depend upon the water for locomotion and food. The most 
notable of these are the muskrat and its enemy the mink. 
(4) Marginal habitats. A subdivision of marginal habitats 
is made regardless of the preceding treatment of the river and 
ereek banks. It has already been hinted at in connection with 
the sorting of materials by the creek. As stated in the general 
discussion at the beginning of the chapter, it is, strictly speaking, 
terrestrial, but. has been classified with the aquatic formations 
because it is the immediate result of the recent action of the 
stream. It consists of mud flats and sand bars. No attempt was 
made to locate these upon the maps on account of the incon- 
staney of their positions in the upper and lower parts of the 
