BIOLOGY OF PHYSA 33 



parts are of peat and sand, while others are clay. The sand 

 and clay have been washed in from the hills, and in places com- 

 pletely hide the substratum of peat. The dry parts of the swamp 

 were searched thoroughly for living water snails, both under 

 debris and in the soil, and with the following results: 



1 . A few Lymnaea palustris and many L. desidiosa were found 



lying under the moist ground in the region of standing 

 water. 



2. In the drier parts, none of these snails were found living 



except in the bottom of cattle footprints which were 

 deep enough to hold sufficient moisture. In one of these 

 moist prints a cat-tail grew and three specimens of L. 

 desidiosa were found sealed to its stem about three or 

 four inches from the bottom of the depression. 



3. Although the two species of Lymnaea were found scattered 



here and there over the swamp, there was no evidence of 

 Physa living or dead, in the dry parts of the swamp, 

 although in the pools of water it vv^as found in abundance 

 feeding upon algae, in company with a very few Lymnaea. 

 Evidently for some reason, when the waters of the 

 swamp began to recede, Physa followed them to their 

 lowest depressions while the other snails did not. 



4. No evidence of burrowing in the soil was seen among the 



mollusks of this swamp wdth the exception of some small 

 bivalves Sphaerium which were found covered with soil 

 to a depth of one to three inches. 



Swamp A\ This swamp differs from A\ in the following 

 respects : The swamp is skirted by clay hills covered with beech 

 and maple ; it has a sparse grow1:h of black ash trees and a medium 

 amount of Hypnum; there is no standing water except in one 

 very small, shallow depression; the substratum is peat; no 

 evidence of erosio 1 except a very little clay washed from the 

 hills at the very border of the swamp. A great variety of snail 

 life W'as present, there being Lymnaea desidiosa, L. palustris, 

 Physa elliptica, P. gyrina hildrethiana, and Planorbis — in addi- 

 tion to Sphaerium. 



Many shells representing all the moUusca of the swamp, 

 except Sphaerium, were strewn over the ground and moss, but 

 only in the two following cases were there any living ones found 



