122 THE GENERAL ANATOMY OF THE CERCOPITHECIDAE [SECT. A 



The Cercopitheci are small monkeys, measuring usually in 

 length some eighteen to twenty inches exclusive of the tail which 

 is characteristically at least as long again as the body of the 

 animal. The tail is not prehensile in the strict sense of that 

 expression, which is properly exemplified by certain monkeys of 

 the New World. 



The body and tail are clad with hair which is of a bright 

 colour and so variegated as to provide an important basis of 

 classification 1 . Individual hairs tend to reveal a banded distri- 

 bution of pigment, a feature common to many groups of the 

 Primates. Tufts of longer hairs may develop on the head, and in 

 some species a definite beard is distinctive. The hind-limbs do 

 not exceed the fore-limbs greatly in length, and in all cases the 

 thumb, though short, is present (it is reduced to the merest 

 remnant in the allied group of Colobus monkeys). The fingers shew 

 a tendency to develop interdigital webs. The head is rounded, 

 the face varying in prominence according to age and sex, for it is 

 relatively largest and most prognathous in aged males. 



The eyes are of large size and darkly pigmented in all varieties. 

 The ears 2 are relatively smaller than in the Lemurs, and the ear- 

 tip is not constantly present. Thus the auricle is involved in a 

 process of reduction, which has affected the genus Cercopithecus 

 more distinctly than the allied genera Macacus and Cynocephalus. 



The nose is flattened and the nostrils are usually separated by 

 a narrow septum only (in the Colobus monkeys the interval between 

 the nostrils may be nearly as wide as in the New World monkeys). 



The possession of large cheek-pouches is a very distinctive 

 feature of many Old World monkeys and the Cercopitheci are 

 thoroughly representative in this respect. They also possess hard 

 callous pads devoid of hair on the buttocks in situations corre- 

 sponding to the tuber ischii of each side. 



1 Perhaps the most remarkable variety is a snow-white form found on the 

 Upper Congo. The skin is as white as the hair, but these monkeys are not 

 albinos, for the eye-pigment is retained in the iris and even on the sclerotic coat. 

 The occurrence of white-skinned monkeys in an area occupied by the most darkly- 

 pigmented Hominidae affords material for free speculation. Here I will only 

 suggest that the dim light of the more dense forests may have contributed to the 

 production of the variety in question. 



- Cf. Keith, Nature, Nov. 7, 1901, pp. 19, 20. 



