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Page:Popular Science Monthly Volume 66.djvu/366

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362
POPULAR SCIENCE MONTHLY.

ON THE RELATIONS OF THE LAND AND FRESH-WATER MOLLUSK-FAUNA OF ALASKA AND EASTERN SIBERIA.
By WILLIAM HEALEY DALL,

SMITHSONIAN INSTITUTION.

HAVING recently completed a census of the land and fresh-water mollusk-fauna of Eastern Siberia and Alaska, together with a discussion of the relations of its elements, it seemed that a summary of the results in their bearing on geographical distribution of animal life in the boreal regions might have some interest for the readers of the Popular Science Monthly.

In the region north of latitude 49° the molluscan population of North America is rather scanty. For distributional purposes it must be divided into two series, one containing the aquatic forms and the other the land shells. The distribution of water animals is carried on by different means from those influential in the dispersal of terrestrial forms, and any discussion which combined the two without distinction would be liable to contain errors of fact and deduction.

The vast territories under consideration have a number of drainage systems which in the tabulation of species should be distinguished. The chief of these are:

1. The Labradorian.—This comprises the area of drainage into Ungava Bay and the Atlantic north of the straits of Belleisle and the 'Height of Land' including the Labrador coast and the northeastern part of the Ungava District of the Dominion of Canada.

2. The Canadian.—This system comprises the drainage of the St. Lawrence and the Great Lakes south and east from the 'Height of Land,' including the island of Anticosti.

3. The Hudsonian.—This, the largest system of all, includes the entire area draining into Hudson Bay, including Keewatin, the southeastern corner of the Mackenzie District, eastern Athabaska, the whole of Saskatchewan, the southeastern two-thirds of Alberta, Assiniboia and Manitoba, the drainage area of the Red river of the north in the Dakotas, and northeastern Minnesota, all of Ontario, Quebec, and Ungava north and west of the 'Height of Land.'

4. The Mackenzian.—This vast system includes the basin drained by the Mackenzie river and its tributaries, covering northwestern Alberta, northeastern British Columbia, and the northwestern two thirds of Athabaska and the Mackenzie District.