Bui. Univ. of Mont., Biol. Series No. 3. 



Plate XXVIL 



A series of shells showing the varieties of Pyramiduhi Strigosa due to high altitude. 

 The largest, to the left, from McDonald Lake, altitude 3300 feet. The next size, from 

 Tobacco Root Mountains, east of the main range. The third, from the Bitter Root Moun- 

 tains, altitude 5000 feet. The smallest, from McDonald Peak, altitude Sooo feet. Photo 

 by M.J. E. 



