BROOK, CREEK, AND RIVER 289 



In the moderately rapid waters with sandy or gravelly 

 bottom will be found Campelonia suhsolidiim and C. integrum, 

 good-sized snails. These spots are the habitat of the clams 

 already mentioned. It is in the larger rivers that the good-sized 

 specie occur abundantly, forming mussel beds on the sand bars. 

 The clam lies buried in the sand or silt with only the posterior 

 portion of the shell protruding. Out of this end stick the 

 siphon tubes which take in the water containing the tiny 

 animals and plants (plancton) on which the clam feeds. It is 

 from the thick shells of the larger species that the pearl buttons 

 are stamped. 



In the quieter stretches of the stream, where the bottom is 



muddy, w^ill be found the burrowing May-fly nymphs of genus 



Hexagenia, the fish and dragon-fly nymphs already noted, and 



.a number of forms identical with those of ponds. Just at the 



^margin of the quiet waters, where the current goes swiftly by, 



rwill be found the May-fly nymph, Chirotenetes siccus. 



The plant life along the shore and in the bed of the stream 

 is in general appearance very like that in similar situations in 

 the ponds and lakes, though the constituent species may be 

 quite different. Attached to the rocks and logs in the stream 

 bed are numerous filamentous algae and some water mosses. 

 Stones and logs are found, slippery with the mucus secreted by 

 diatoms or other algae that coat their surfaces with a dense 

 layer of plant life, and the stream is often choked in its quieter 

 stretches with submerged plants similar to those of the ponds, 

 Myriophylum, Utricularia, Elodea, Vallisjieria, Cera.topJiyllum, etc. 

 In quiet stretches pond lilies are common. The shores are lined 

 with zones of plants, rushes, cat-tails, arrow root, sedges, and 

 grasses. 



These relatively quiet marginal areas are the breeding-places 

 for an abundant plant and animal life, largely minute forms, the 

 overflow of which is carried downstream in the current as the 

 plancton. This consists largely of microscopic plants, diatoms, 

 desmids, and other single-celled forms, of some filamentous types, 



