XVII CHARACTERS OF ANTHROPOIDEA 355 
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 Catar- 
rhines are invariably thirty-two in number, the formula being 
12 Ci Pm? M3=32. 
In the Old-World Apes there is a bony external auditory 
meatus, which is wanting (as a bony structure) in the Platyr- 
rhines. 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 pre- 
molars 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 
