OF NORTH AMERICA. 27 



so that they do not become dry. Then again, the weathering of the 

 rock has left bowl-shaped hollows a centimeter or so across and about 

 the same depth, and the snails usually get into them. 



"To summarize, the known facts bearing on the distribution of 

 these four species are as follows : 



"1. Their lower limit is 4.5 to 5 dm. depth of water, governed 

 possibly by the water pressure or the food supply. 



"2. The upper limit is, for Limncea sanct&marice and Limncea 

 emarginata, 1.5 dm. of water, for Physa sayii and Physa sp., the shore- 

 line. The cleaner the rock and the less the wave action, the shallower 

 the water which they may inhabit. 



"3. Their horizontal distribution is controlled by (a) full exposure 

 to the sun; (b) a rock bottom; (c) a certain minimum of wave action." 



These examples of environmental conditions might be vastly in- 

 creased in number, but enough have been given to illustrate the con- 

 ditions under which this group of mollusks live. 



The Lymnseas are able to live under many and diverse conditions. 

 In altitude they have been recorded from sea-level to a height of 10,000 

 feet or more; they also live in fresh, brackish and in salt water, in 

 thermal springs and in ice-cold waters, at or near the surface of the 

 water and at great depths in the largest and deepest lakes. 



Some of the records of the altitudes at which Lymnaeas have been 

 found are given below : 



LYMN^A STAGNALIS. 



Outlet Lake Tahoe, Placer County, California, 6247 feet altitude (Cooper). 



GALBA OBRUSSA. 



La Grulla, San Pedro Mountains, 7000 feet altitude (Nelson and Godman). 



GALBA BULIMOIDES. 



Outlet Lake Tahoe, Placer County, California, 6247 feet altitude (Cooper). 



GALBA HUMILIS MODI CELL A. 



Big Payette Lake, Idaho, 5000 feet altitude (Ashmun). 



GALBA PALUSTRIS. 



Bear Lake, San Bernardino Mountains, 6700 feet (Stephens). 

 San Bernardino Mountains, Bluff Lake, 7500 feet altitude (Berry). 

 Walker Lake, San Francisco Mountains, Coconino County, Arizona, in 

 crater of extinct volcano, 8250 feet altitude (Merriam). 



Goshen, near Boulder, Boulder County, Colorado, 8000 feet altitude (Hen- 

 derson). 



