SUBALPINE MOLLUSCA. 8$ 



tion in the whorls as represented in his figure, but we 

 find also every degree of variation in their irregularity 

 that seems possible, while some are normal. The figures 

 also show how much variation occurs in proportions of 

 different specimens. Besides this, in examining any 

 large number of specimens of any species, we find some 

 in which more or less irregularity of one or more whorls 

 exists. //. flexata is stated to agree in every other re- 

 spect with //. trivolvis Say, and the figure shows no other 

 differences. It is therefore evident that it is nothing 

 more than a variety, and considering its occurrence 

 chiefly in subalpine regions (his being found only at 

 9,700 feet elevation in the Rocky Mountains), we may 

 safely attribute it to debility in the animal, caused in these 

 lofty localities by insufficient heat of the water, at some 

 time during the animal's growth. In the irregular growths 

 found in lowland examples, a deficiency of food or 

 some impurity of the water may be a probable cause. It 

 must be remembered that the animal crawls with the shell 

 vertically supported on its back, and thus a condition 

 of debility will allow the weight of the shell to incline it 

 to one side, and its growth, by additions at the mouth, 

 becomes changed in direction, thus altering the plane. 

 In several species we observe this deflection of the mouth 

 taking place just before the maturity of the shell, when it 

 is probably weakened by age, and so constant is this in 

 some of them that it is called a specific character. 



Mr. Ingersoll found //. trivolvis normally developed 

 up to 8,000 feet elevation, and Mr. Raymond also found 

 that the number of deformed shells was least near Hetch- 

 Hetchy Valley, 4,100 feet, more at Lake Eleanor, 4,600 

 feet, and constant at Soda Springs, 8,700 feet. In all 

 cases they inhabited only shallow weedy ponds, none 

 beinor in the lake or rivers. These being fed from melt- 



