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) (ggc}, 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) (101), 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 (Ischnura 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 of 

 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 



