12 JEAN DAWSON 



deserted river bed habitat, but it is in a late stage of filling 

 and contains many water weeds and few Physa. 



2. At Geddes, Michigan, the Huron river has overflowed 

 several acres of flat country, much of which remains under 

 water the 3'ear around, a general view of which is shown in 

 Fig. 7. This "overflow" was examined for Physa, but the snail 

 was found only in the small ponds which had little or no direct 

 communication with the river. Here Physa w^ere crawling upon 

 a mass of Ceratophyllmn covered by about a foot of water. 

 The peculiar feature of this habitat is that the snails are not 

 restricted to a few inches around the margin of the pond, but 

 extend six feet or more out from shore. They evidently use 

 the water plants as a substratum as they are not found beneath 

 them. The result is as though the snail lived in a very shallow 

 pond of but a few inches in depth. With the exception of a 

 few such rather isolated ponds this overflow is practically 

 barren of snails. Few water weeds grow anywhere where the 

 water is directly connected with the river and everything in- 

 dicates the presence of the muddy load of the river. 



3. West of the mill race at Ann Arbor, in the spring flood 

 of the river some pools were left which became completely 

 captured by Char a, Ceratophyllmn, and Elodea. Very few 

 Physa are found in them and these are very small and 

 young. 



4. Discussion of the River System as a Habitat. The river sys- 

 tem does not offer very favorable conditions to Physa in spite 

 of the fact that the little Railroad Creek east of Ann Arbor 

 has very favorable conditions in some parts. The conclusion 

 is reached, however, that such a combination of conditions 

 as is seen in this particular habitat is very rare. The apparent- 

 ly favorable physical conditions found in streams are frequently 

 seen but usually they are found in combination with others 

 which tend to exclude Physa from the locality. The physical 

 conditions found in the optimum itself are subject to 

 change, which will undoubtedly sooner or later (judging 

 from the places already cited having like conditions) , cause a 

 scarcity of Physa in the region that is now so very favorable 

 for them. Thus the removal of the stones from the creek bed 

 would cause this region to offer no shelter from the current 

 and it would become like the region above and immediately 



