This sub-region serves to connect together the other three, since they all merge gradually into it; while to the north it passes into the circumpolar zone which is common to the Palæarctic and Nearctic regions. The greater portion of it is an extensive forest-district, mostly of coniferæ; and where these cease towards the north, barren wastes extend to the polar ocean. It possesses several northern or arctic forms of Mammalia, such as the glutton, lemming, reindeer, and elk, which barely enter the more southern sub-regions; as well as the polar bear and arctic fox; but it also has some peculiar forms, and many of the most characteristic Nearctic types. The remarkable musk-sheep (_Ovibos_) is confined to this sub-region, ranging over a considerable extent of country north of the forests, as well as Greenland. It has been extinct in Europe and Asia since the Post-pliocene epoch. Such purely Nearctic genera as _Procyon_, _Latax_, _Erethizon_, _Jaculus_, _Fiber_, _Thomomys_, and _Hesperomys_, abound, many of them ranging to the shores of Hudson's Bay and the barren wastes of northern Labrador. Others, such as _Blarina_, _Condylura_, and _Mephitis_, are found only in Nova Scotia and various parts of Canada. About 20 species of Mammalia seem to be peculiar to this sub-region. _Plate XX. Illustrating the Zoology of Canada._--We have here a group of Mammalia characteristic of Canada and the colder parts of the United States. Conspicuous in the foreground is the skunk (_Mephitis mephitica_), belonging to a genus of the weasel family found only in America. This animal is {136}celebrated for its power of ejecting a terribly offensive liquid, the odour of which is almost intolerable. The skunks are nocturnal animals, and are generally marked, as in the species represented, with conspicuous bands and patches of white. This enables them to be easily seen at night, and thus serves to warn larger animals not to attack them. To the left is the curious little jumping mouse (_Jaculus hudsonius_), the American representative of the Palæarctic jerboa. Climbing up a tree on the left is the tree porcupine (_Erethizon dorsatus_), belonging to the family Cercolabidæ, which represents, on the American continent, the porcupines of the Old World. In the background is the elk or moose (_Alces americanus_), perhaps identical with the European elk, and the most striking inhabitant of the northern forests of America, as the bison is of the prairies.