_Mammalia._--Four genera seem to be absolutely confined to this sub-region, _Nectogale_, a peculiar form of the mole family (Talpidæ); _Poephagus_, the yak, or hairy bison of Thibet; with _Procapra_ and _Pantholops_, Thibetan antelopes. Some others more especially belong here, although they just enter Europe, as _Saiga_, the Tartarian antelope; _Sminthus_, a desert rat;, and _Ellobius_, a burrowing mole-rat; while _Myospalax_, a curious rodent allied to the voles, is found only in the Altai mountains and North China; and _Moschus_, the musk-deer, is almost confined to this sub-region. Among the characteristic animals of the extreme north, are _Mustela_, and _Martes_, including the ermine and sable; _Gulo_, the glutton; _Tarandus_, the reindeer; _Myodes_, the lemming; with the lynx, arctic fox, and polar bear; and here, in the Post-pliocene epoch, ranged the hairy rhinoceros and Siberian mammoth, whose entire bodies still remain preserved in the ice-cliffs near the mouths of the great rivers. Farther south, species of wild cat, bear, wolf, deer, and pika (_Lagomys_) abound; while in the mountains we find wild goats and sheep of several species, and in the plains and deserts wild horses {218}and asses, gazelles, two species of antelopes, flying squirrels (_Pteromys_), ground squirrels (_Tamias_), marmots, of the genus _Spermophilus_, with camels and dromedaries, probably natives of the south-western part of this sub-region. The most abundant and conspicuous of the mammalia are the great herds of reindeer in the north, the wolves of the steppes, with the wild horses, goats, sheep, and antelopes of the plateaus and mountains. Among the curiosities of this sub-region we must notice the seal, found in the inland and freshwater lake Baikal, at an elevation of about 2,000 feet above the sea. It is a species of _Callocephalus_, closely allied to, if not identical with, one inhabiting northern seas as well as the Caspian and Lake Aral. This would indicate that almost all northern Asia was depressed beneath the sea very recently; and Mr. Belt's view, of the ice during the glacial epoch having dammed up the rivers and converted much of Siberia into a vast freshwater or brackish lake, perhaps offers the best solution of the difficulty.[9]