Owing to the fact that the former Republic of Mexico comprised much territory that belongs to the Nearctic region, and that many Nearctic groups extend along the high-lands to the capital city of Mexico itself, and even considerably further south, there is much difficulty in determining what animals really belong to this sub-region. On the low-lands, tropical forms predominate as far as 28° N. latitude; while on the cordilleras, temperate forms prevail down to 20°, and are found even much farther within the tropics. _Mammalia._--Very few peculiar forms of Mammalia are restricted to tropical North America; which is not to be wondered at when we consider the small extent of the country, and the facility of communication with adjacent sub-regions. A peculiar form of tapir (_Elasmognathus bairdi_) inhabits Central America, from Panama to Guatemala, and, with _Myxomys_, a genus of Muridæ, are all at present discovered. _Bassaris_, a remarkable form of Procyonidæ, has been included in the Nearctic region, but it extends to the high-lands of Guatemala. _Heteromys_, a peculiar genus of Saccomyidæ or pouched rats, inhabits Mexico, Honduras, Costa Rica, and Trinidad. Five genera of monkeys extend here,--_Ateles_, _Mycetes_, _Cebus_, _Nyctipithecus_, and _Saimiris_; the two former alone reaching Mexico, the last only going as far as Costa Rica. Other typical Neotropical forms are _Galera_, the tayra, belonging to the weasel family; _Nasua_, the coatimundi; _Dicotyles_, the peccary; _Cercolabes_, the tree porcupine; _Dasyprocta_, the agouti; _Cælogenys_, the paca; _Choloepus_, and _Arctopithecus_, sloths; _Cyclothurus_, an ant-eater; _Tatusia_, an armadillo; and _Didelphys_, oppossum. Of Northern forms, _Sorex_, _Vulpes_, _Lepus_, and _Pteromys_ reach Guatemala. _Birds._--The productiveness of this district in bird life, may {53}be estimated from the fact, that Messrs. Salvin and Sclater have catalogued more than 600 species from the comparatively small territory of Guatemala, or the portion of Central America between Mexico and Honduras. The great mass of the birds of this sub-region are of Neotropical families and genera, but these are intermingled with a number of migrants from temperate North America, which pass the winter here; with some northern forms on the high-lands; and with a considerable number of peculiar genera, mostly of Neotropical affinities.