ECOLOGY OF ISLE ROYALE. 61 



of 10-40 cm. Along the south side of a large island near the south shore, 

 where they are sheltered from waves in every direction, the same spe- 

 cies is abundamt. They live on rocks in the full sun in water 10 — 45 cnn 

 deep, Avith the optimum depth at 20 — 25 cm. They may occur on the 

 tops or sides of rocks, but never on the sand between them. Associated 

 with the Limnaeu, but much less abundant, are Planorhis hicarinatus 

 royalensis Walker (No. 210), P. campanulatus Say (Nos. 210, 211), 

 Lampsilis luteolus (Lam.) (Nos. 210, 211), Anodonta nmrginata Say 

 (No. 210), and Anodonta grandis footiana Lea (Nos. 210, 211). 



Opportunity was given to observe the behavior of Limnaea stagnalis 

 (No. 217) in waves of some size near a small circular island half a kilo- 

 meter out in the lake. The bottom was gently sloping, and either of 

 solid rock or of large rounded fragments. There were no overhanging 

 trees, so the shells were found in uniform abundance in the usual depth 

 of water on all sides of the island. At the time the island was visited 

 a strong wind was blowing, and the waves were probably nearly as high 

 as they ever become on Siskowit Lake. One or two shells were seen 

 which had been washed loose, and of course would be unable to re- 

 attach themselves until the waves abated. It would be expected that in 

 such cases the shells might be crushed or broken or the animal killed. 

 That such may happen was evidenced by finding a few live shells which 

 had been cracked and then healed, leaving an irregular surface. Their 

 occurrence here and elsewhere only upon rocks of considerable size 

 shows that they require a firm substratum, and where the rocks are 

 free from any coating of slime they can certainly endure higher waves.' 

 Around the island under discussion the rocks were washed perfectly 

 clean. 



The beach in front of the camp at Siskowit Bay {Fig. 29) was inhabited 

 by large numbers of (No. 200) Physa sayii Tapp., PJiysa sp., Limnaea 

 stagnalis L., and Limnaea emarginata Say, so that more detailed obser- 

 vations of them could be made, and a few experiments carried out to 

 show their sensitiveness to the depth, or bathytropism, as it has been 

 termed. The beach here is of rock with a gentle slope of about one in 

 five, corresponding to the dip, except where blocks have worn off, leaving 

 low vertical walls. The wave action here is very light, its force being 

 cut off by a series of islands lying between the beach and the main body 

 of Siskowit Bay. This was well shown by the conditions on August 2, 

 when there was scarcely a ripple inside the islands, although the bay 

 outside was covered with whitecaps. The beach is covered with a thin 

 coat of slime formed mostly of excrement from the snails. 



On such a beach snails may live close to the edge of the water, but 

 the larger Limnaeas still occupy their usual depth of 1.5 to 4.5 deci- 

 meters. About 10 A. M., on August 3, all the shells to a depth of about 

 1 decimeter were gathered from a strip of the beach about 10 meters 

 long. They were comprised in the following species: (No. 200) Limnaea 

 stagnalis L., Limnaea emarginata Say, Physa sayii Tapp., and Physa sp. 

 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 spe- 

 cies, had migrated upon the same strip. The only evidence concerning 

 the way that they came is that one shell of Limnaea stagnalis was seen 

 to drift up over a low wall into the shallow zone. This method could 



