The Monkeys are primarily divisible into two great divisions, which have been termed, on account of the characters of the nose,
the Catarrhines and Platyrrhines. In the former the nostrils look downward and are close together; in the latter they are separated by a broad cartilaginous septum, and the apertures are directed outwards. But numerous other points of difference separate these two groups of the Monkey tribe. The Catarrhines often have those remarkable ischial callosities, patches of hard skin brightly coloured; the tail may be totally wanting as a distinct organ, as is the case, for instance, with the Anthropoid Apes; there are often cheek pouches, so that, as Mr. Lydekker has remarked, if a Monkey be observed to stow nuts away in its cheeks for future reference, we may be certain that its home is in the Old World, for the Catarrhines are exclusively denizens of the Old World, while the Platyrrhines are as exclusively New World in range. Again, those of the Catarrhines which do possess a long tail, such as the members of the genus Cercocebus, never show the least sign of prehensility in that tail. The teeth of the Catarrhines are invariably thirty-two in number, the formula being I 2/2 C 1/1 Pm 2/2 M 3/3 = 32.
In the Old-World Apes there is a bony external auditory meatus, which is wanting (as a bony structure) in the Platyrrhines. The late Mr. W. A. Forbes pointed out that in most of the New-World forms the parietals and the malars come into contact; in the Monkeys of the Old World they are hindered from coming into contact by the frontals and the alisphenoids. The Platyrrhines may have the same number of teeth; this is the case with the Marmosets, but in them there are three premolars and two molars; in the remaining New-World Monkeys there are thirty-six teeth, but of these three are premolars and three molars.
Not only are these two groups of the Primates absolutely distinct at the present day, but they have been, so far as we know, for a very long time, since no fossil remains of Monkeys at all intermediate have been so far discovered. This has led to the suggestion that the Monkeys are what is termed diphyletic, i.e. that they have originated from two separate stocks of ancestors. It is hard, however, to understand on this view the very great similarities which underlie the divergences that have just been mentioned. But, on the other hand, it is equally hard to understand how it is that, having been separated from each other for so long a period, they have not diverged further in
structure than they have. The Platyrrhines seem to stand at the base of the series. This is another example of the existence of archaic creatures in South America.
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.