Occasional Papers of the Museum of Zoology 17 



• Habitat ip. Szvarnp near Rock Dam. A few rods east of 

 the rock ridge at this place a small hollow had in the past 

 held a little bog, grown up with tamaracks, spruces, alders and 

 willows, but the fire of the preceding summer had destroyed 

 all of the trees and burnt away the humus. This depression 

 still gathered little ponds after the heavier rains, but the 

 destruction of the humus allowed the water to penetrate the 

 soil more rapidly, and the loss of the trees increased the rapid- 

 ity of evaporation. For this reason it had ceased to be a 

 favorable locality for aquatic shells, and all of the specimens 

 obtained were dead and burnt. 



SPRING BROOKS (TABI.E; III) 



Habitat 20. Spring Brooks. All through this region are 

 numerous small spring brooks, which almost invariably flow 

 from the hardwoods on to the sandy, outwash plains. These 

 streams are usually only a foot or two wide, and have a soft 

 bottom composed of decaying plant material, except in places 

 where a swift current keeps the muck washed away and 

 exposes the rocky or sandy substratum. None of the streams 

 investigated showed signs of molluscan life, with the excep- 

 tion of one stream which crosses the road to Foster City, about 

 two miles north of Hancock Creek. 



This brook has a sandy and pebbly bottom near the culvert. 

 Here Planorbis parvus and juvenile specimens of Physa zval- 

 keri were found clinging to the pebbles. 



Habitat 21. Fern Creek. This is a small, quite swift stream 

 which flows from between the granite ridges into Pine Creek, 

 in the Menominee Trough. The bottom is composed of gravel 

 and stones, except in a few places where mucky-bottomed 

 pools are formed; in most places the stream is covered over 

 by fallen trees, etc. 



