THE XALTILLS. 6 



Large diam., 7-10 mm., average of ten specimens, 8.91 mm.; 

 greatest depth, 3-5 mm., average of ten, 4.34 mm.; aperture, 3.65— 

 4.38 mm., average of eight, 3.99 ; whorls, 4-4.50 mm., average of ten, 

 4.26 mm. 



Specimens taken at 8,500 feet, on Sinyaleamin mountain. Mission 

 Range, Montana. Also taken on McDonald Peak, same range. Alt. 

 7,800 to 8,500. Types at the University of Montana. 



The averages from the seven localities where shells have been col- 

 lected show very conclusively the effect of altitude on the size of the 

 shells. Increase in altitude diminishes the length of the season, the 

 amount of heat received, the amount of food supply, and the chances 

 of life. Tlie result is to stunt or dwarf the animals attaining the 

 heights. This is plainly shown in the sizes of shells at the different 

 altitudes. As greater altitudes are reached, shells reduce in diameter, 

 in depth, in the size of the aperture and in the number of whorls. 

 Young specimens taken from the adults at Flathead Lake had shells 

 with 2.25 to 2.50 whorls. If all the young at different altitudes start 

 with the same number when born, the reduction of shell growth in 

 spirals is easily deduced. The very significant observation is that a 

 few hundred feet in altitude shows a corresponding reduction in size 

 of the shells. The smallest shells are but three-eighths the diameter 

 of the largest, one-third of the depth, have an aperture two-fifths as 

 large, and have but two-thirds the number of whorls. The relative 

 proportions of the largest, from Flathead Lake, to the smallest, on 

 Sinyaleamin mountain, are seen from the following approximate 

 ratios : 



Largest shells. Smallest shells. 

 Large diam. to depth . . . 11 to 7 14 to 7 



Depth to width of aperture . . 28 to 22 20 to 22 



Large diam. to aperture . . 23 to 11 21 to 11 



This story, in brief, as brought out by study, is as follows : Pyra- 

 midula strigosa., var. cooperi, from some source got into the Flathead 

 Lake region. At this altitude, 3,000 feet, it flourished and grew, but 

 the slow-moving animals migrated. As they ascended the mountain 

 sides, following the streams to the banks of the lakes, and then as- 

 cended the wooded slopes the difficulties in securing food for exist- 

 ence became more of a problem. The shorter season required more 

 hardy animals. Stunting or accidental variation produced smaller 

 individuals, which would not require so much food on account of the 



