20 JEAN DAWSON 



2. On the south and narrower portion of the lake, bars are 



being built parallel to the shore. 



3. The southwest shore is filling in by an invasion of the sedges. 



4. Spits are closing off small bays on the east shore. 



5. Bars just beginning to be formed tangential to the shore 



in several places on the east side of the lake, are marked 

 by a growth of bulrushes. 



6. Partially formed bars are gradually elongating and will 



soon reach the opposite point of the shore. 



7. A spit is being formed from a small island in the center 



of the lake to the shore which will connect the island 

 with the mainland, 



Physa was sought in the open shore water, behind the different 

 spit and bar formations, and in the waters that were wholly 

 cut off from the lake. A very characteristic distribution of 

 the snail was found in the different habitats which varied with 

 the dominance of certain physical elements occurring there. 

 The optimum habitat (Fig. 9) was found behind a spit some 

 20 to 30 feet oft' shore, which ran tangential to the southeastern 

 shore of the lake. The spit is being built on its free end and in 

 the near future will join the opposite shore. The snails are not 

 at all evenly distributed even in this narrow strip of water, 

 although it is so nearly enclosed b\^ the spit. Physa heter- 

 ostropha Say, and Lymnaea desidiosa Say are found by hundreds 

 in this optimum habitat, for a distance of about 15 feet along 

 the shore and about four to six feet back from the water's edge. 

 In Fig. 9 this locality is shown opposite to the small white square 

 near the center of the foreground. This clearly defined area 

 is situated far enough back from the open end of the spit to 

 be beyond the reach of severe wave action, and yet near enough 

 to allow a free exchange of well aerated lake water. In point 

 of numbers it supports a remarkable snail life. Loose soil 

 has washed into this habitat from the neighboring ploughed 

 field in such quantities as to fill in the shelving shore and to 

 kill the plant life except for a moderate growth of Chara and 

 algae. Numbers of both Physa heterostropha and L. desidiosa 

 are crawling in this habitat in waters some of which are so 

 shallow that the snails are not wholly immersed. 



The number of snails behind this spit diminished from this 

 thickly peopled optimum in the following manner: The number 



