364 POPULAR SCIENCE MONTHLY. 



Labrador and the Hudsonian region, but in the main, the molluscan 

 population of the waters of the region described may be regarded as 

 a northward extension of the Northeast American and Mississippi Val- 

 ley faunas, diminished by the elimination of all the less hardy species 

 and reinforced by the addition of a small number of circumboreal 

 forms. The total fresh-water fauna comprises 138 species, of which 

 90 occur in the Hudsonian system, but only 23 extend to the Mackenzie 

 basin, though the Yukon system may boast of 29. Some eccentricities 

 of distribution will be referred to later. 



The distribution of land shells is not affected to any serious extent 

 by rivers or streams, but is doubtless, in the main, the result of the 

 slow movement of individuals. 



The pulmonate fauna of Alaska is composed of four elements; con- 

 tributions from the faunas of Asia, of the Pacific Coast region of 

 America, of the Hudsonian or Canadian region, and of the special cir- 

 cumboreal or common subarctic fauna of the whole northern hemis- 

 phere. 



Differences of latitude mean for the snail not so much differences 

 of temperature corresponding to the latitude, as differences of the 

 annual active period, which diminishes as one proceeds northward. 

 Snails at Point Barrow (and there are such) must remain in a state 

 of hibernation at least nine months in the year, and I suspect that this 

 more probably brings a limiting strain on the vitality of the organism, 

 than would the mere occurrence at times of a specially low tempera- 

 ture. 



The Alaskan land shells number 86 species, of which 16 are cir- 

 cumboreal, and the same number (though not all the same identical 

 species) are common to the northeastern part of Siberia. Fifty spe- 

 cies are held in common with the Pacific fauna, while 34 are part 

 of the Yukon fauna which is so intimately allied to that of the Hud- 

 sonian region. These latter two groups are limited by topographic 

 features. Thus the fauna of the Hudsonian region, in constantly 

 diminishing number of species, is extended to the northwest through 

 the Yukonian region north of the Coast range and the Alaskan range, 

 to Bering Sea on the west and the Arctic coast on the north. 



In like manner the fauna of the Columbian region of the Pacific 

 Coast, is extended along the Alaskan coast and islands south and 

 west of the two ranges mentioned, and between them and the sea. 

 Some of the most characteristic and larger species are cut off in their 

 northwestern extension by the area near Mount St. Elias, where along 

 one hundred miles of coast immense glacier fields extend to the very 

 border of the sea. The last representatives of this fauna disappear 

 among the eastern Aleutian islands. In British Columbia a few 

 species belong to the Valley region between the Rocky and the Cascade 

 Mountains, and do not reach the sea coast, but these are too few to 



