28 JEAN DAWSON 



Lake behind bars and in small lagoons that are entirely cut off 

 from the lake. The water that is cut off from the river, as the 

 overflow at Geddes, Fig. 7, is more or less filled with debris. 

 In the former practically no snails live, but in the latter there 

 were many snails living upon the plants with several inches of 

 clear water above them. 



III. Hypnum Habitat. The best Hypnum habitats were found 

 in swamps more or less shaded and having stagnant water. The 

 old cedar swamps of Dayton, Tuscola County, Mich., furnish 

 the optimum examined for this plant, although luxuriant beds 

 of it were found in the ox-bow pond of Lima Center Creek and 

 in small pools in Ka^'anaugh swamp. Where Hypnum grows in 

 luxuriance, Physa is not found, but where there is a moderate 

 amount of it, the snails are found in numbers. 



IV. Elodea Habitat. There is an Elodea habitat in the little 

 Railroad Creek where the stream is cutting a new channel in 

 the bottom of its old one. Most of the water is flowing in the 

 new bed but there is also a sufficient amount of water to cover 

 the old bed to a depth of eight to ten inches. The deeper, swifter 

 water has no plant life, but in the shallow quiet water of the 

 old bed there is a great abundance of Elodea. So completely 

 has it captured these quiet waters that no other plant is seen 

 and it grows in such abundance that it is crowding above the 

 water surface in places, as may be seen in the foreground of 

 Fig. 5. No sign of Physa was found, either living or dead, in 

 this habitat although it was present in habitats where Elodea 

 was growing in less abundance a few yards farther up stream. 

 An analysis of the optimum Elodea habitat and a comparison 

 of it with the above described optimum creek bed habitat of 

 Physa which lies but a few rods above it, serves to show the 

 influence which water plants may play when they are present 

 in great abundance. The Elodea habitat has the following con- 

 ditions: Shallow water ; full sunlight; quiet waters, perhaps more 

 or less mingled with the flowing creek waters especially on the 

 margin; very little decaying debris; no enemies observed; and 

 an abundance of water plants. The only perceptible difference 

 between the Elodea habitat and that of the creek-pool habitat 

 is that there are more water plants and perhaps less currents 

 flowing over them, yet this region was carefully searched with- 

 out finding a single specimen of Physa. Other places in the 



