16 JEAN DAWSON 



sedge formations of lakes and a study made of the conditions 

 under which it was found living. 



No Physa, either living or dead, are found in the shallow 

 water along the shores of Lake Michigan at Frankfort, Mich. No 

 visible plant or animal life is found in this shallow disturbed 

 water, and the pebbles and stones are worn smooth by the waves 

 which unceasingly break upon the shore. It is evident that 

 Physa could not live in such conditions, as it occurs only in pro- 

 tected places. The lake in the vicinity of Frankfort is bounded 

 by high clay bluffs which the waves are actively cutting; con- 

 sequently there is a narrow, severely wave swept beach in time 

 of storms. On such a beach there is no chance for the forma- 

 tion of pools or swamps which might harbor Physa. 



Crystal Lake at Frankfort is cut off from Lake Michigan by a 

 neck of land about one-fourth of a mile wdde. This lake is about 

 nine miles long and three miles wide, and is surrounded by high 

 morainic hills. Living Physa were sought in vain in the shal- 

 low waters along its shores, but some bleached and weather- 

 beaten shells of Physa ancillaria Say and P. heterostropha Say 

 together with the shells of some other genera of fresh-water 

 mollusks were frequently found scattered in the shallow water, 

 and upon the dry sand of the shore. A diligent search in these 

 waters proved them to be as barren of visiVjle plant and animal 

 life as was the same zone in Lake Michigan. The pebbles are 

 worn and smooth and bespeak the power of the waves that 

 break upon the shore. 



a. The Beach Pool as a Habitat. The beach of this lake is 

 wider than that of Lake Michigan, just described, and this is 

 due to its water level having been lowered several years ago, 

 by an outlet cut to Lake Michigan through Betsie River. Upon 

 this wide beach are found depressions in the sand filled with 

 ground water and water dashed up from the lake when the sea 

 is high, yet they are far enough from the water's edge to be out 

 of danger from too strong wave action. These pools were well 

 populated with living Physa ancillaria Say, P. heterostropha Say 

 and other genera of snails. The dead and bleached shells found 

 in the shallow water of the lake and on the dry sand were prob- 

 ably washed from these beach pools in time of severe storm 

 and died upon the sand or w^ere killed by being tossed about 

 by the waves. 



