22 JEAN DAWSON 



have a supply of lake water which backs up into them in times 

 of high water or storm. This number is no doubt greatly lessened 

 by the amount of decaying leaves which fall into the water from 

 the bushes and trees which skirt the lake shore. Along the east 

 shore where a spit is just beginning to be built, a few snails and 

 some dwarfed Char a may be seen. The shallow water on un- 

 protected shores presents conditions very much like the waters 

 of the same kind at Kavanaugh and Crystal Lakes, and is equally 

 barren of life. 



Passing over in review the spit and bar-formed habitats of 

 Crooked Lake it is found that the waters are either so sheltered 

 that they have become foul with decaying debris, or that the 

 spits are too young to offer sufficient protection from wave- 

 action. There is but one habitat which lies between these two 

 extremes and offers both protection from waves and well aerated 

 water and that is the optimum above described. 



e. Open Shores of Small Lakes as a Habitat. This habitat 

 occurs in the smaller, comparatively shallow lakes, which by 

 their natural setting have a greater or less amount of shore 

 which is protected from the sweep of the wind and waves and 

 which offers a foothold to Physa. Long Lake, Benzie County, 

 Mich., and the south shore of the Third Sister Lake, near 

 Ann Arbor, furnish examples of this sort of habitat. 



2. Discussion of the Lake as a Habitat. In general still 

 water offers much more favorable conditions for Physa than 

 does that of flowing water. Not only do the shallow marginal 

 waters of the lake harbor the snail whenever protection from 

 too violent wave action occurs, but in many cases the shores 

 themselves offer very favorable conditions for it, (Gleason 1905 

 pp. 60-62). This observation agrees with that of Wetherby (1881 

 p. 158) who says: "First we may consider the circumpolar distri- 

 bution of Lymnaeidae. These molluscs are essentially lacustrine, 

 for while they are distributed into rivers and smaller streams 

 to some extent, their station of fullest development is in lakes 

 the world over." 



Once knowing the conditions under which the snail lives in 

 these lake waters it is well to consider the forces which are at 

 work modifying these conditions and the resulting effect upon 

 the future of Physa in these localities. It is not probable that 

 the snails can ever find sufficient protection from the severe 

 wave action of such lakes as Michigan and Superior as to enable 



