LYMN^HLE OF NORTH AMERICA. 25 



them. Associated with the Limncea, but much less abundant, are 

 Planorbis bicarinatus royalensis Walker, P. campanulatus Say, Lamp- 

 silis luteolus (Lam.), Anodonta marginata Say, and Anodonta grandis 

 footiana Lea. 



"Opportunity was given to observe the behavior of Limncea sanc- 

 tcemarice 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 reattach 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 rinding 

 a few live shells which had been cracked and then healed, leaving 

 an irregular surface. Their occurrence here and elsewhere only upon 

 rocks of considrable 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 was inhabited 

 by large numbers of Physa sayii, Physa sp., Limncea sane tee mar ice and 

 Limncea cmarginata, so that more detailed observations 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 

 decimeters. 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 : Limncea 

 s an dec mar ice , Limncea cmarginata, Physa sayii Tapp., and Physa sp. 



