112 MR. R. I. rOCOCK ON THE EXTERNAL 



these genera is the organ provided with a baculum (os j'>enis). In 

 Cebus, however, the terminal portion of the penis is strengthened 

 with a baculum as in the Catarhine Monkeys, and the tip of the 

 bacuk^m forms a small prominence in the middle of the subcir- 

 cular terminal area of the penis, thrusting the orifice to tlie right 

 of the middle line. For the rest the penis of Cebus differs from 

 that of Ateles in the gradual expansion of its distal portion up to 

 the truncated tip. (Text-fig. 13, J, N.) 



The Tail. 



I have no new facts to add to those already recorded about 

 the tails of South American Monkeys. In the four genera of 

 Hapalidse {Hapale, Mystax, CEdipomidas, and Leontocehits) and 

 in six of the genera of Cebidse (Callimico, Callicebus, Saimiris, 

 Fithecia, Gacajao, and Aotus) the tail is evenly bushy or hairy 

 throughout and not prehensile. Only in Cebidae of the genera 

 Cebus, Lagothrix, Brachyteles, Ateles, and Alouatta is it prehensile. 

 Since, therefore, the tail is prehensile in only fifty per cent, of the 

 genera of South American Monkeys and in a minority even of 

 the Cebidse, the prevalent idea, derived from the text-books, that 

 caudal prehensility is characteristic of Platyrhine Primates is 

 indefensible. 



In nearly all the species of Cebus the tail, although prehensile, 

 is evenly hairy to the tip above and below. Only in the Central 

 American form (C. capucinus) is the end naked below. In this 

 genus practically the only modification of the tail is the develop- 

 ment of the flexor muscles at the expense of the extensors, so 

 that the organ is normally curled in its distal half. So far, there- 

 fore, as the tail is concerned, Cebus bridges the interval between 

 such genera as Saimiris or AoUos, for example, and the genera 

 Lagothrix, Brachyteles, Ateles, and Alouatta, in which the prehen- 

 sile power and tactile sensibility of the tail reach a maximum.' 

 In the latter genera the distal portion of the tail is highly mobile 

 and sensitive, with the skin of its lower side naked and trans- 

 versely ridged, like the underside of an Elephant's trunk (text- 

 fig. 7, C). It serves, indeed, the purpose of an additional hand 

 not only for climbing, but for grasping objects beyond the reach 

 of the arms*. I am unable to satisfy myself, however, that this 

 special modification of the tail is proof or even strong evidence of 

 afiinity between the genera that exhibit it. Ateles and Brachy- 

 teles are probably closely allied ; but Lagothrix and Alouatta 

 differ from both very markedly in the structure of the hand ; 

 and Alouatta differs at all events from Lagothrix and Ateles in its 

 stenorhine nostiils, its ears and cranial characters. 



* I have seen a Spider-Monkey {Ateles) in the Gardens thrust its tail tln'ougli the 

 bars of its cage and take food from visitors which it could not reach with its hand. 

 Capuchins {Cehus) apparently never attempt this; but I have seen them use their 

 tails for carrying paper bags and other things, ver^' much as the Kat Kangaroo 



{Bettongia) carries tufts of grass. 



