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over the United States. Species of Vitrina, Zonites, Pupa, and Succinea, are found in Greenland; and Eastern Palaearctic species of Vitrina, Patula, and Pupa occur in Alaska. More than 30 species of shells living in the Eastern States, are found fossil in the Post-Pliocene deposits of the Ohio and Mississippi. Fresh-water Shells.-North America surpasses every other part of the globe in the number and variety of its fresh-water mollusca, both univalve and bivalve. The numbers up to 1866 were as follows:—Melaniadae, 380 species; Paludinidae, 58 species; Cycladidae, 44 species; and Unionidae, 552 species. The last family had, however, increased to 832 species in 1874, according to Dr. Isaac Lea, who has made them his special study; but it is probable that many of these are such as would be considered varieties by most conchologists. Many of the species of Unio are very large, of varied forms, and rich internal colouring, and the group forms a prominent feature of the Nearctic fauna. By far the larger proportion of the fresh-water shells inhabit the Eastern or Alleghany sub-region; and their great development is a powerful argument against any recent extensive submergence beneath the ocean of the lowlands of North America.
The Nearctic Sub-regions.
The sub-divisions of the Nearctic region, although pretty clearly indicated by physical features and peculiarities of climate and vegetation, are by no means so strongly marked out in their zoology as we might expect. The same genera, as a rule, extend over the whole region; while the species of the several sub-regions are in most cases different. Even the vast range of the Rocky Mountains has not been an effectual barrier against this wide dispersal of the same forms of life; and although some important groups are limited by it, these are exceptions to the rule. Even now, we find fertile valleys and plateaus of moderate elevation, penetrating the range on either side; and both to the north and south there are passes which can be freely traversed by most animals during the summer. Previous to the glacial epoch there was probably a warm period, when every part of the range supported an abundant and varied fauna, which, when the cold period arrived, would descend to the lowlands, and people the country to the east, west, and south, with similar forms of life. The first, and most important sub-division we can make, consists of the Eastern United States, extending across the Mississippi and the more fertile prairies, to about the 100°th. meridian of west longitude, where the arid and almost desert country commences. Southwards, the boundary bends towards the coast, near the line of the Brazos or Colorado rivers. To the north the limits are undefined; but as a considerable number of species and genera occur in the United States but not in Canada, it will be convenient to draw the line somewhere near the boundary of the two countries, except that the district between lakes Huron and Ontario, and probably Nova Scotia, may be included in the present sub-region. As far west as the Mississippi, this was originally a vast forest country; and it is still well wooded, and clothed with a varied and luxuriant vegetation. The next, or Central sub-region, consists of the dry, elevated, and often arid district of the Rocky Mountains, with its great plateaus, and the barren plains of its eastern slope; extending northwards to near the commencement of the great forests north of the Saskatchewan, and southward to the Rio Grande del Norte, the Gulf of California, and to Cape St. Lucas, as shown on our maps. This sub-region is of an essentially desert character, although the higher valleys of the Rocky Mountains are often well wooded, and in these are found some northern and some western types. The third,or Californian sub-region, is small, but very luxuriant, occupying the comparatively narrow strip of country between the Sierra Nevada and the Pacific. To the north it may include Vancouver's Island and the southern part of British Columbia, while to the south it extends to the head of the Gulf of California. The fourth division, comprises the remainder of North America; and is a country of pine forests, and of barren wastes towards the Arctic Ocean. It has fewer peculiar species to characterise it than any other, but it possesses several characteristic arctic forms, while many of those peculiar to the south are absent; so that it is a very convenient, if it should not be considered an altogether natural, sub-region. We will now give an outline of the most important zoological features of each of these divisions, taking them in the order in which they are arranged in the Fourth Part of this work. California comes first, as it has some tropical forms not found elsewhere, and thus forms a transition from the Neotropical region.
I. The Western or Californian Sub-region.
This small district possesses a fruitful soil and a highly favourable climate, and is, in proportion to its extent, perhaps the richest portion of the continent, both zoologically and botanically. Its winters are far milder than those of the Eastern States in corresponding latitudes; and this, perhaps, has enabled it to support several tropical forms which give a special character to its fauna. It is here only, in the whole region, that bats of the families Phyllostomidae and Noctilionidae, and a serpent of the tropical family, Pythonidae, are found, as well as several Neotropical forms of birds and reptiles.
Mammalia.-The following genera are not found in any other part of the Nearctic region. Macrotus (Phyllostomidae), one species in California; Antrozous (Vespertilionidae), one species on the West Coast; Urotrichus (Talpidae) one species in British Columbia; sub-genus Nesorew (Soricidae), one species in Oregon; Bassaris (Procyonidae), California; Enhydra (Mustelidae), Pacific Coast; Morunga (Phocidae), California; Haploodon (Haploodontidae) a rat-like animal, allied to the beavers and marmots, and constituting a peculiar family found only in California and British Columbia. The following characteristic Nearctic forms also extend into this sub-region —Tawidea, Procyon, Didelphys, Sciuropterus, Tamias, Spermophilus, Dipodomys, Perognathus, Jaculus.
Birds.-Few genera of birds are quite peculiar to this subregion, since most of the Western forms extend into the central district, yet it has a few. Glaucidium, a genus of Owls, is confined (in the Nearctic region) to California; Chamara, a singular form allied to the wrens, and forming a distinct family, is quite peculiar; Geococcyx, a Neotropical form of cuckoo, extends to California and Southern Texas. The following genera are very characteristic of the sub-region, and some of them almost confined to it: Myiadestes (Sylviidae); Psaltriparus (Paridae); Cyanocitta, Picicorvus (Corvidae); Hesperiphona, Peucara, Chondestes (Fringillidae); Selasphorus, Atthis (Trochilidae); Columba, Melopelia (Columbidae); Oreort/e (Tetraonidae). Reptiles.—The following genera are not found in any other part of the Nearctic region: Charina (Tortricidae); Lichanotus (Pythonidae); Gerrhonotus (Zonuridae); Phyllodactylus (Geckotidae); Anolius and Tropidolepis (Iguanidae). Seeloporus (Iguanidae) is only found elsewhere in Florida. All the larger North American groups of lizards and snakes are also represented here; but in tortoises it is deficient, owing to the absence of lakes and large rivers. Amphibia.-California possesses two genera of Salamandridae, Aneides and Heredia, which do not extend to the other subregions. Fresh-water Fish.-There are two or three peculiar genera of Cyprinidae, but the sub-region is comparatively poor in this group. Plate X VIII. Illustrative of the Zoology of California and the Rocky Mountains.—We have chosen for the subject of this illustration, the peculiar Birds of the Western mountains. The two birds in the foreground are a species of grouse (Pediocates Columbianus), entirely confined to this sub-region; while the only other species of the genus is found in the prairies north and west of Wisconsin, so that the group is peculiar to northern and western America. The crested birds in the middle of the picture (Oreortyr picta), are partridges, belonging to the American subfamily Odontophorinae. This is the only species of the genus which is confined to California and Oregon. The bird at the top is the blue crow (Gymnokitta cyanocephala), confined to the Rocky Mountains and Sierra Nevada from New Mexico and Arizona northwards, and more properly belonging to the Central