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Saki are all inhabitants of various localities within this district. The mode of distribution of the three species of Ouakari is still more remarkable. Each of them, as first shown by Bates and afterwards further explained by Forbes, is limited to a comparatively small tract of forest on the banks of the Amazon and its affluents. The Black-headed Ouakari (B. melanocephatus), as shown by the accompanying map (prepared by Forbes), which we
have been kindly permitted to use by the Zoological Society of London, is met with only in a tract traversed by the Rio Negro, the Bald-headed Ouakari appears to be confined to the triangle formed by the union of the Amazon with another affluent, the Japurà, and the Red Ouakari to the forests on the north bank of the Amazon opposite Olivença, and lying between the main stream and the river Iça. Each of them evidently takes the place of the others in its particular district. Of this peculiar kind of distribution few instances are known amongst mammals, but many somewhat similar cases have been observed in birds, reptiles, and butterflies.
The fourth and lowest sub-family of the Cebidæ (Nyctipithecinæ) includes three genera—the Douroucoulis (Nyctipithecus), the Titis (Callithrix), and the SquirrelMonkeys (Chrysothrix), and numbers altogether some twenty species. This group, as a whole, has a wide range like the two first sub-families, extending from Central America to Paraguay. But the species are most abundant in the centre of the area, where some of them, so far as is yet known, have a very limited range.
SECTION V.-DISTRIBUTION OF THE MARMOSETS
Like the Platyrrhine Monkeys the little Marmosets, which constitute the family Hapalidæ, are entirely restricted to the tropical forests of the New World. The family embraces but two genera generally acknowledgedHapale with about seven, and Midas with about fourteen or fifteen species. But these small creatures are still little known, and it is probable that many more of them remain to be discovered when the vast forest-region through which they are distributed shall have been more thoroughly explored. The Marmosets do not extend so far north as the true Monkeys, only a single species (Midus geoffroyi) having yet been ascertained to range north of the Isthmus of Panama as far as Chiriqui. Thence, southwards, they are thinly distributed over the South American continent down to the northern provinces of the Argentine Republic, where Hapale penicillata is said to occur in the forests of Salta
and Jujuy. The Marmosets appear to be most numerous in the forests of Amazonia, where some of them are confined to very restricted districts. Our best account of these animals is to be found in the journals of the excellent observer Bates.
SECTION VI.-SUMMARY AND DEDUCTIONS
The subjoined table contains a list of the genera of the Order Quadrumana," or Monkeys, and gives the approximate number of species met with in each of the great geographical Regions.
1. The Order Quadrumana, or Monkeys, contains about 212 species, divisible into twenty genera and four families.
2. Monkeys are found only in the tropical and subtropical portions of the Eastern and Western Hemispheres, and are absent in Australia and Madagascar.
3. The Monkeys of the Eastern and Western Hemispheres are quite distinct in structure, and belong to different families.
4. In the Western Hemisphere Monkeys are restricted to the Neotropical Region, not occurring north of 20° N. lat., or south of 30° S. lat.
5. In the Eastern Hemisphere some few species of Monkeys are met with as far north as 40° N. lat., and in Africa descend to 35° S. lat.
6. The Monkeys of the Ethiopian and Oriental Regions belong to distinct genera.
7. The Anthropoid Apes are restricted to the tropics of Africa and Asia, the most highly organized of them being the Chimpanzees of Africa and the Orangs of Asia.
SECTION VII.-DISTRIBUTION OF THE LEMURS
The Lemurs, which by many recent authorities are united with the Monkeys to form the Order Primates, but which it is, in some respects, more natural to retain as an Order by themselves, number altogether some fifty species-only onefourth of the number of the Quadrumana. They are also very different in geographical distribution, thirty-five out of the whole series being confined to the Malagasy Sub-region, whilst the few remaining forms are met with onlyin isolated portions of the Ethiopian and Oriental Regions. Besides the family of