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
