With regard to the Monkeys of the New World, they are to be grouped and classified as follows:--The Howlers must be placed by themselves, then the Spider Monkeys; the Lagothrix and the Sajous form a very distinct group; and thus the prehensile-tailed series is complete. Then come the non-prehensile-tailed. The Sakis form one group, and the Squirrel Monkeys, and the Night, or Owl Monkeys (the Douroucoulis), make a second. The Arctopithecini are another family, and consist of the Marmosets and Tamarins. _Family._ _Sub-Family._ _Genus._ _Example._ { Mycetes Howler. Platyrhini, { or Cebidæ Prehensile-tailed { Ateles Spider Monkey. { Lagothrix Barrigudo. { Cebus Cai. { Pithecia, { including { Brachyurus Saki. Non-prehensile-tailed { Callithrix Squirrel Monkey. { Nyctipithecus Douroucouli. Arctopithecini Hapale Marmosets. Midas Tamarins. The American Monkeys present some remarkable instances of the localisation and dispersion of species; allied kinds of different species, but with the same habits, occupying neighbouring districts, or being rather remote. And it is noticed that the great rivers form barriers between the homes of different kinds, which, however, mingle at the river source, and in the country not rendered impassable to them by broad streams. Thus Wallace noticed that the Howler (_Mycetes Beelzebub_) is apparently confined to the Lower Amazon, in the vicinity of Para, and a black species to the Upper Amazon, the Red Ursine Howler having the Rio Negro and the Upper Amazon as its forest ground. One Spider Monkey is found only in the Guiana district north of the Amazons, and another, the _Ateles ater_, inhabits West Brazil, but the species of the genus range, as a whole, over the forest regions from the south of Mexico to 30° south latitude, and even on the west of the Andes.