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 Valley 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; contributions from the faunas of Asia, of the Pacific Coast region of America, of the Hudsonian or Canadian region, and of the special circumboreal or common subarctic fauna of the whole northern hemisphere.
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 temperature.
The Alaskan land shells number 86 species, of which 16 are circumboreal, and the same number (though not all the same identical species) are common to the northeastern part of Siberia. Fifty species 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 Hudsonian 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
