Group I. Platyrrhina

Fam. 1. Hapalidae.—We may begin the account of the Platyrrhine Monkeys with the Hapalidae or Marmosets; for this family is structurally lower than the rest. They have thirty-two teeth, arranged as in the following formula: I 2/2 C 1/1 Pm 3/3 M 2/2 = 32. The molars have three main tubercles, and not four as in the higher forms. The digits are for the most part clawed, not nailed, as in the higher types; the great toe alone bears a flat nail. The tail, too, is ringed, a condition which is characteristic of many of the lower groups of mammals, but not of the higher Apes. The cerebral hemispheres are smooth, but this is a matter rather connected with their small size than with low zoological position. The tails of the Marmosets, unlike those of so many other American Monkeys, are not prehensile though they are long.

The genus Hapale is broadly distinguished from the other genus, Midas, by the fact that the lower incisors slant forwards as in the Lemurs. They are small, soft-furred, long-tailed Monkeys, familiar to every one. There are some seven species, which are entirely restricted in range to Brazil, Bolivia, and Colombia, one species only, H. pygmaea, extending northward into Mexico.

Of Tamarins, genus Midas, there are rather more species—about fourteen. They are South and Central American in distribution. Since both these genera are arboreal in habit, it is extraordinary that they have not the prehensile tails of their American allies. As, however, the late Mr. Bates observed an individual of the species M. nigricollis fall head-foremost from a height of at least 50 feet, alight on its feet, and run off as if nothing in particular had occurred, it is evident that no extra prehensile powers are absolutely necessary. Some of the Tamarins have a long mane; this is well seen in M. rosalia, or rather in M. leoninus, which, if not identical with it, is at least very closely allied to it. The name is obviously derived from the character referred to, and the Monkey, originally described by the traveller von Humboldt, is said to have "the appearance of a diminutive lion." M. bicolor is an example of the species with no mane, but with a patch of white round the mouth, looking like "a ball of snow-white cotton" held in the teeth.

Fam. 2. Cebidae.—The remaining American Monkeys are comprised in the family Cebidae. This is to be distinguished from the last by the fact that there is an additional molar, thus making thirty-six teeth in all. The tail, sometimes very short, is more generally long and highly prehensile, being nude at the extremity, which part is therefore especially prehensile; this state of affairs is often to be seen in animals with prehensile tails. The Cebidae, though for the most part larger than the Marmosets, never approach in size the Old-World Apes.

Typical of the family is the genus Cebus, including the "Capuchin" Monkeys, and consisting of nearly twenty species; the tail, though prehensile, is covered with hair to the tip, a fact which is indicative of a less perfect prehensility than is exhibited in some Monkeys with a naked under surface to the tip of the tail. The thumb is well developed. The genus ranges from Costa Rica to Paraguay. The commonest Monkey which accompanies the street organs of this country is a Cebus. It is a popular delusion that these and other monkeys are purely vegetable-feeding animals. Cebus is in fact particularly fond of caterpillars, as are also the Marmosets.

Allied to Cebus is Lagothrix, the Woolly Monkey, of which L. humboldti is the best-known species, there being indeed but one other. It is a larger and heavier animal than any species of Cebus; and the Hare-like woolliness of the fur suggested its scientific name to its original describer, von Humboldt. It has a perfectly prehensile tail, naked at the tip. The thumb and great toe are well developed. These are purely fruit-eating Monkeys, and are known as "Barrigudos" by the Portuguese of the Amazon country on account of their prominent belly, due apparently to the immense amount of fruit consumed. They are, or were, much eaten by natives.

Brachyteles is a little-known genus, connecting the last with the next genus. The under fur is woolly; the thumb is small or absent. The tail is naked below.

The Spider monkeys, Ateles or Coaitas, have been described as the most typically arboreal of American monkeys. The use of the prehensile tail can frequently be studied in living examples in the Zoological Society's Gardens. With this "fifth hand" the Monkey feels for a place to grasp, and securely twists its tail round, moving it with the greatest ease from point to point. When the tail is being thus used it is carried erect over the head. The fact that this genus possesses no functional thumb is thought to be associated with the extreme perfection of its adaptability to an exclusively arboreal life. The hand without a thumb can act as an equally efficient hook for suspending the body; and what is useless in nature tends to disappear. These Monkeys have a wide range, extending from Mexico in the north to Uruguay in the south. There are ten species. The flesh of many Monkeys is eaten not only by natives but by Europeans; but the Spider Monkeys are said to furnish the most sapid food of all.

Fig. 265.—Spider Monkey. Ateles ater. × 1⁄12.

The Howling Monkeys, genus Mycetes, have also received the appropriate generic names of Alouatta and Stentor. The former of these two names, indeed, is that which should properly be applied to the genus. But Mycetes is perhaps better known. The "howling" is produced by saccular diverticula of the larynx, larger than those of other American Monkeys, such as Ateles, where, however, they are also developed. The hyoid bones, too, are enormously enlarged and cavernous, while the jaw—in order to accommodate and protect these various structures—is unusually large and deep. The Howlers are furnished with a fully prehensile tail. The thumb is present. They are described as being the most hideous in aspect of the American Monkeys, and of the lowest intelligence, with which latter characteristic is associated a less convoluted brain than in Ateles, for example. The noise produced by these Monkeys is audible for miles, and is said not to be due to emulation, i.e. not to be comparable to singing or talking, but to serve to intimidate their enemies. The story told of these and other Monkeys with prehensile tails, that they cross rivers by means of a bridge of intertwined Monkeys, is apparently devoid of truth. There are six species, which are Central and South American in range.

The Squirrel Monkeys, genus Chrysothrix, are small creatures with a long head, the occiput projecting. Their tail, though long, has no naked area at the extremity and is non-prehensile. It is a remarkable fact that the proportions of the cranium as compared with the face are greater, not only than in other Monkeys, but than in Man himself. The thumb is short, but not so short as in the Spider Monkeys. The cerebral hemispheres are very smooth; but, as already remarked, this is a matter of size, and not of low position in the series. It may appear at first sight that this statement contradicts the one made concerning the Howlers. But the latter are large Monkeys, and therefore ought, so to speak, to have a more complex brain; but they have not. Like so many of the American Monkeys, the Squirrel Monkeys are gregarious, and, in spite of their tails, arboreal. They are largely insect-feeders, and also catch small birds and devour eggs. There are four species, of which C. sciurea is the commonest, and is constantly an inmate of the Zoological Society's Gardens. Humboldt asserted of it that when vexed its eyes filled with tears; but Darwin did not succeed in seeing this very human expression of an emotion.

Callithrix is a genus not far removed from the last, and, like it, occurs both in Central and in South America. It is chiefly to be distinguished from Chrysothrix by the non-extension backwards of the head, and by the more furry character of the tail. The lower jaw is rather deep, as in the Howlers; but there is not, or there has not been discovered, a howling apparatus like that of Mycetes. Nevertheless Professor Weldon has found in a female of C. gigot a patch of ossification on the thyroid cartilage of the larynx which may be an indication of something more in the male. There are eleven species.

Nyctipithecus, the Doroucouli Monkeys, is a genus of somewhat Lemurine appearance, caused by their large eyes. But they reminded Bates of an Owl or a Tiger-cat! They have a long, but not prehensile tail. As in the Marmosets, the lower incisors project forwards in a Lemurine fashion. The thumb is very short. A peculiarity of this genus is the twenty-two dorso-lumbar vertebrae. As in Chrysothrix, but not as in Callithrix, the hemispheres of the brain are smooth. There are five species, of which one occurs so far north as Nicaragua; the rest are Brazilian, extending down to the Argentine.

Fig. 266.—Red-faced Ouakari. Brachyurus rubicundus. × 1⁄5.

The Ouakari Monkeys, Brachyurus, are, as the name denotes, short-tailed forms. Two species, B. rubicundus and B. calvus, have bright red faces; B. melanocephalus has a black one. There is a small thumb. The brain is fairly convoluted, and is to be specially compared with that of Cebus and Pithecia. The species B. rubicundus at any rate has an absolutely as well as a relatively greater length of intestines and caecum than any other American Monkey known.

Fig. 267.—White-nosed Saki. Pithecia albinasa. × 1⁄5. (From Nature.)

Not the least remarkable fact about these Ouakari Monkeys is their distribution in South America. We cannot do better than quote the summary given by Messrs. P. L. and W. L. Sclater in their Geography of Mammals, which is as follows: "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. melanocephalus) ... 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 Japura; 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 insects."

The genus Pithecia, the Sakis, consists of five species with long bushy tails, which are non-prehensile. They are bearded and have a thumb. Like the last genus, Pithecia does not extend into Central America. The incisors project forwards, and the lower jaw is deep, though the howling apparatus of Mycetes is wanting. The thin, closely-set, and projecting incisors are very suggestive of those of the Lemurs. Brachyurus is much like Pithecia in this respect, and both differ markedly from such a genus as Cebus, where the lower incisors are vertical. An anatomical peculiarity of Pithecia is the breadth of the ribs. P. satanas is perhaps the best-known species, but all five have been exhibited at the Zoological Society's Gardens. As its name suggests, P. satanas is entirely black; it shows a curious point of difference from P. cheiropotes in its way of drinking. The latter species, as its name denotes, uses its hand to drink, while P. satanas puts its mouth to the water. P. albinasa is black with a red patch on the nose, within which again is a small white patch.