XX.-THE MOLLUSK FAUNA OF THE PRIBILOF ISLANDS. 



By William H. Ball. 

 (With a map.) 



The west coasts of America, taken together in both hemispheres, so far as the 

 moUusks are concerned might be regarded as forming one great faunal region, modi- 

 fied only by the influence of temperature. In practice it is divided into three regions 

 between the Polar faunas : The Peruvian, whose northern limit is usually placed at 

 Cape Blanco, and whose exteiit roughly coincides with the Coast washed by the 

 Peruvian current; the Mexican or Fanamic, which extends from Cape Blanco north- 

 ward to Point Conception, California, though until lately supposed to be much more 

 restricted; and the Oregonian, which includes the coast northward from Point Con- 

 ception to and including the Aleutian Islands and that part of the basin of Bering 

 Sea south of the area covered by pack ice in winter. The Pribilof group stands on 

 the very northern verge of the Oregonian region. The latter is conveniently subdi- 

 vided into three provinces or subregions : The Californian, from Point Conception to 

 Oap.e Mendocino, California; the Oregonian proper, thence northward to Mount St. 

 Elias or Yakutat Bay; and the Aleutian, extending thence westward to and including 

 the Aleutian or Catherina Archipelago and the Commander Islands. The most north- 

 ern outpost of the Aleutian province is the Pribilof group. The faunal conditions of 

 Bering Sea are somewhat peculiar and require a particular description. They are 

 dependent upon bathymetrical and thermal factors. Contrary to the traditions of 

 the text-books, as I have elsewhere' shown at length, the chief current of this sea is a 

 drift of cold water southward, which is particularly marked along the Kamchatkan 

 coast. On the northern border of the Pacific, south of the peninsula of Alaska and 

 the Aleutian Islands, the tides rise toward the northward and westward, while a 

 reflected branch of the Japan current or Kuro Si wo, which is deflected northward from 

 the vicinity of Dixon entrance, in north latitude ^4°, feebly reenforces the action of the 

 tides and at flood pours through the passes between the islands during summer a 

 certain amount of water having a temperature between 45° and 50° P., which has a 

 westerly set. This flow endures only for about a third of the twenty- four hours, and 

 is so feeble that west of west longitude from Greenwich 170° it can not be discrimi- 

 nated from the ordinary flow of the rising tide. A few fathoms below the surface the 

 water temperature rarely exceeds 40° ¥., which is the mean summer temperature of 



'Notes on Alaska aud the vicinity of Bering Strait; Am. Journ. Soi., 3d ser., XXI, pp. 104-111, 

 with map, February, 1881; and Appendix No. 16, Rep. U. S. Coast aud Geodetic Survey for 1880; The 

 Currents and Temperatures of Bering Sea, 46 pp., 4°, 2 maps and section, Washington, March, 1882. 



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