104 ANIMAL COMMUNITIES OF STREAMS 
grandis). All of the animals of the silt formation burrow and prob- 
ably require little oxygen. 
Planorbis bicarinatus formation, or formation of the vegetation: 
Here we have for the first time the conditions which we find in ponds— 
a dense rooted vegetation. With such a growth of vegetation we have 
a very different fauna: a large number of aquatic insects and pulmonate 
(lunged) snails. Of these there are a considerable number of species 
which must come to the surface of air, both in the adult and the young 
stages. The most important of these are the bugs: water scorpions 
(Ranatra fusca), the creeping water-bugs (Pelocoris femoratus), the small 
water-bug (Zaitha fluminea), the water-boatmen (Corixa sp.), the still- 
water brook beetles or parnids (Elmis quadrinotatus), several species 
of predaceous diving beetles (Dytiscidae) (99c), and water scavengers 
(Hydrophilidae). The pulmonate snails are Physa integra, Planorbis 
bicarinatus (Figs. 62, 63), and often species of Lymnaea. 
Where the bottom is not too soft we often find numbers of viviparous 
snails (Campeloma) and an occasional mussel (Anodonta grandis). The 
crustaceans are distinctly clear-water forms: the crayfish (Cambarus 
propinquus) (tor), the amphipod (Hyalella knickerbockeri), and the 
brook amphipod (Gammarus fasciatus) (102). 
The gilled aquatic insects are the May-fly nymphs (Caenis and 
Callibaetis sp.) and the damsel-fly nymphs (/schnura verticalis) and 
dragon-fly nymphs (Aeschnidae and Libellulidae). To practically all of 
these the vegetation is necessary as a resting-place or clinging-place, or a 
place to enable them to creep to the surface to shed the larval skin and 
become adult. 
Variations of the formation: The Fox is fairly representative oj 
base-level rivers beyond the reach of tide-water except perhaps that the 
presence of gravel and sand in this stream may not seem fully in accord 
with this statement. There are, as has been noted, rivers near Chicago 
in which these conditions, which go along with old age in a stream, are 
still more marked. The lower Deep River is perhaps a good example of 
this. It is very sluggish and the bottom in the vicinity of Liverpool, 
Ind., is, so far as we have been able to ascertain, entirely covered with 
silt, with considerable humus mixed with it. The margins are peaty. 
The Calumet and the lower Black are similar. In these, sand and 
gravel areas, and animals which inhabit them, are reduced to a mini- 
mum and the silt and vegetation associations are better developed. 
Characters of the formation: The vegetation formation is distinct 
and clearly marked off from all others. The animals are dependent upon 
