8 JEAN DAWSON 



strikes the Successive stones in its path in a similar manner. 

 The protection given by these stones to the stream bottom 

 depends upon the size of the stones and the distance between 

 them. 



Feeding upon the Cladophora and crawling upon the gravel 

 on the bottom of this creek are myriads of Physa gyrina; hun- 

 dreds of these snails can be picked up in the space of a few 

 feet. At first sight they seem to be crawling without regard 

 to the swift current, but upon closer examination they are 

 found to be living on the sheltered sides of the stones and on 

 the sheltered creek bottom. In general the number of snails cor- 

 responds with the density of the growth of the plant on the 

 stones. The thickly massed Cladophora is crowded with snails, 

 while the shorter and less crowded plants have correspondingly 

 fewer snails. 



The conditions of this habitat are: (a) a gently sloping shore 

 and shallow water throughout (a slope of about i inch to the 

 foot) ; (b) running water, well aerated and having no decaying 

 vegetable debris; (c) protection from the current by stones; 



(d) a growth of Cladophora, which furnishes abundant food; 



(e) no gro\\1:h of large pond weeds such as Chara and Cerato- 

 phyllum; (f) full sunlight; (g) a stable substratum. 



Above this optimum habitat, and opposite to the two trees 

 on the bank in Fig. 3, is a place that is somewhat similar to the 

 optimum just described but which differs from it, in that its 

 banks are steeper and its bed narrower. In consequence the 

 water is deeper and the current swifter. A small number of 

 snails are found feeding upon Cladophora, on the more pro- 

 tected sides of the stones, but more are living on the long grass 

 which trails into the water from the creek banks. Other in- 

 stances of the snail living in the creek bed, and apparently 

 braving the current, are found in this region of the creek. For 

 instance, where an old shoe or tin can lies near the shore in 

 the shallow water specimens of Physa gyrina are found living 

 on their protected sides. 



Considerable time was spent in examining this creek in detail, 

 but it did not furnish another favorable creek bed habitat. 

 For the greater part of its length, the creek has cut a wide, deep 

 bed in which it flows less swiftly than at the optimum pre- 

 viously described. In a few cases however, it presents areas 



