[Illustration: FIG. 73.—Opossum (_Didelphis marsupialis_).—By permission of the N.Y. Zoölog. Soc.] The most characteristic portion of North America, zoölogically speaking, is the Sonoran region of Dr. Merriam, the Warm Temperate of Dr. Allen. It crosses the continent from ocean to ocean, its northern boundary following for most of the way the 43d parallel of latitude, but over the Great Plains and Great Basin, on each side of the Rocky Mountains and the high plateaus, it extends to lat. 48°. On the south, it takes in the greater part of Mexico, covering all of the table-land of that country, the lowlands of which belong to the South American or Neotropical region. The Sonoran is invaded from the north by the long branches from the Boreal and Transition zones, which follow the three great mountain-systems in the manner already explained, and the Mexican plateau permits the similar invasion of Neotropical territory by the Sonoran fauna. Characteristic Sonoran genera, none of which extend into the Boreal, are the opossums (_Didelphis_), in the southern part a peccary (_Tagassu_) or “Wild Texas Pig,” representative of a family of swine quite different from the true pigs of the Old World, and an armadillo (_Tatu_). A very isolated form is the Prong-horned Antelope (_Antilocapra americana_); there are several species of the typically American deer (_Odocoileus_) which differ in important respects from those of the eastern hemisphere, and the Bison was very abundant until exterminated by Man. Bison, antelope and deer also reach or extend into the Boreal zone, but the former, or Wood Bison, is probably a different species from the plains animal. [Illustration: FIG. 74.—Prong-horned Antelope (_Antilocapra americana_).—By permission of the N.Y. Zoölog. Soc.] [Illustration: FIG. 75.—Kangaroo-Rat (_Dipodomys philippii_).—By permission of the N.Y. Zoölog. Soc.] [Illustration: FIG. 76.—Thirteen-lined Spermophile (_Spermophilus tredecimlineatus_).—By permission of the N.Y. Zoölog. Soc.]