

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 Limnea sancte- 
marie 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 Limnea emarginata and Limnea 
sanctemari@, 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 Limnzeas 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 Limnea 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, 
