26 THE CHICAGO ACADEMY OF SCIENCES. 



The smaller Physas were especially abundant and about 200 of them 

 were taken. Four hours later, at 2 p. m., 60 shells, all of the smaller 

 species, had migrated upon the same strip. The only evidence con- 

 cerning the way that they came is that one shell of Limncea sanctce- 

 marice was seen to drift up over a low wall into the shallow zone. This 

 method could hardly account for 60 of the smaller ones, however, 

 appearing in so short a space of time. It may be taken as indicating 

 a general and continued migration in all directions within their bathy- 

 tropic limits. 



"It was noticeable that the large Limn&a emarginata and Limncea 

 sanctamarice , aside from the one specimen mentioned above, live at 

 an average depth of 3dm. and never deeper than 4.5 or 5 dm. To 

 test their bathytropism, six of them were picked out of the deeper 

 water by hand and held in contact with the bottom in the shallow zone 

 until they extended their feet and attached themselves. At this time 

 the water was very quiet, moving just enough to cause a faint sound 

 on the beach. But the size of the shell of the two Limnseas is so large 

 that they offer considerable surface to the water and are consequently 

 easily washed loose. Two of the six swung a little from side to side 

 and were then washed off and carried by the undertow into water 3 

 dm. deep, where they again attached themselves. A third, without 

 being shaken by the waves, clung to the rock for some time, then 

 suddenly let go of its hold and drifted over a low ledge into the 

 deeper water. Two others immediately started to crawl down the 

 slope, and one in about fifteen minutes, the other in about half an hour, 

 had crawled over the ledge into water 3 dm. deep, where they both 

 remained stationary. The sixth remained attached, and in three hours 

 had crawled 2 dm. parallel to the shore, keeping at the same depth. 

 The next morning, twelve hours later, it had disappeared and, of 

 course, could not be recognized in the deeper water. 



"On August 4 two shells of Limncea appeared in the shallow zone, 

 but it is not known whether they drifted or crawled up. They were 

 there at least three hours. After they were last observed a fresh 

 breeze sprang up from the east and the slight wave action caused by 

 it probably washed them down. 



"The level of the lake varies somewhat with the direction and 

 intensity of the wind, so that in front of the camp a strip of beach 

 up to 5 dm. in width may or may not be covered with water. The 

 smaller shells, Physa sayii Tapp. and Physa sp., live in this zone in 

 spite of the fact that they are sometimes out of water. So far as 

 observed, they are never exposed for any considerable length of time, 



