Page:Popular Science Monthly Volume 66.djvu/370

366 There is one local species, Eulota weyrichi, known only from Sakhalin Island; and another, Helicigona subpersonata from the valley of the Ud. Three forms of Vivipara (of which two are probably variants of Chinese forms) are the only exclusively local species of the vast Amur Valley, or drainage, not known from other regions. Nine specially Kamchatkan species have been described, but about half of them are doubtfully distinct.

The total number of land and fresh-water mollusks known from the Amurland, Sakhalin, Kamchatka, the Chukchi peninsula, and the Asiatic coast north of the Amur and east of the Stanovoi range, is only eighty-one. Of these thirteen are circumboreal species, and twelve are supposed to be locally peculiar. Of the remainder

Of these erratic species a few may be especially mentioned. Margaritana margaritifera, as is well known, is absent from the whole of the great northern central region of North America, though it appears in the Lower Saskatchewan, the sources of the Missouri and in eastern Canada, while on the Pacific it ascends at least to latitude 56° N. In eastern Asia it is known from Kamchatka, Sakhalin Island, the upper portion of the Amur basin, and southern Mongolia—but I find no authoritative record of it thence westward to Northern and Middle Europe. Schrenck did not find it on the lower Amur.

Phisa fontinalis is reported from the upper Amur and (in a duck's crop) the desert of Dauria, but is not known from Siberia proper, though common in Europe. There is an entire absence of typical Physa throughout East Siberia, so far as reported; and only one species of Ancylus or Unio is known from east of the Yenisei river of Siberia.

Aplexa hypnorum is known from Northern Europe, Western Siberia and the Chukchi peninsula, but has not been reported from Eastern Siberia, or the Amur, though abundant in Alaska, and reaching on the Taimyr peninsula to 73° 30' North Latitude.

Zoögenites harpa is known from Northeastern Scandinavia in Europe; from Northeastern America, the Hudson Bay territory and Southeastern Alaska, in America; but in Siberia it is recorded only from the easternmost margin; the Chukchi peninsula, Bering Island, Kamchatka and the lower Amur. These singularities of distribution must await much more extended knowledge before they can be adequately discussed, but it is believed that to some extent they are due to the transgression of the sea, or of glacial ice, over part of the area in which a species might naturally be expected to occur, thus delaying the occupation of the entire region by the species concerned.