100 MR. II. I. rOC'OCK ON TUE EXTERNAL 



The hands and feet of the typical Oebidae while exhibiting an 

 interesting range in structural variation — e. g., in the develop- 

 ment of the pads, the relative lengths and spacing of the digits as 

 described below under the different genera — have certain features 

 in common, which may be briefly referred to by way of 

 introduction. 



The talons are always narrow and compressed and not infre- 

 quently acuminate, but are never so strongly compressed, curved, 

 and pointed at the tip as in the Hapalidse and Callimico. As in 

 the Hapalidfe, the pollex, when present, is a short edition of the 

 other fingers, and is never truly opposable to them even to the 

 extent that it is in the Old World Monkeys, being set much 

 closer than in the latter to the base of the second digit, although 

 the space between them varies to a certain extent according to 

 the genera. The hallux is typically well developed, although 

 somewhat reduced in Ateles. It can be extended at right angles 

 to the long axis of the foot, and it projects approximately from 

 the middle of the side of the latter, near-ly half-way, that is to 

 say, between the second digit and the tip of the heel. 



The digits of both hands and feet, apart from exceptional cases, 

 are free from webbing — that is to say, they are separated almost 

 down to tlie plantar pad when viewed from the lower side. 

 Digits 3 and 4 both on the hands and feet are frequently sub- 

 equal ; and since they are occasionally subequal in Lemurs and 

 in the Catarhine Monkeys, there is in this respect a complete 

 gradation between the Lemurs, in which digit 4 typically sur- 

 passes 3, and the Catarhine Primates, in which digit 3 typically 

 surpasses 4. 



The plantar and digital pads are as a rule not well defined, but 

 in Aotus they are especially well developed, and they surpass the 

 average in Scdmiris. 



In Ceh^is the palm of the hand is tolerably long and digits 2, 

 3, 4, and 5 are subequally spaced ; digit 1 (the pollex) is com- 

 paratively long, and a little further i-emoved from digit 2 than 

 the latter is from digit 3. The foot is much longer than the 

 hand. (Text-fig. 6, A, B.) 



The hands and feet of CalUcehus do not appear to differ 

 materially from those of Cehus. (Text-fig. 4, E, F.) 



Scdmiris has the pads better defined than in either of the 

 preceding, and the palm of the hand is relatively broader. 

 (Text-fig. 4, C, D.) 



In Aotus the hand is also relatively broader ihan in Cehus and 

 CalUcehus, and the pads are exceedingly well developed and 

 coarsely striate. It seems probable that the exceptional develop- 

 ment of the pads and sensory strise in this genus is an adaptation 

 to the nocturnal habits of this Monkey, the specialised tactile 

 sense compensating for imperfect nocturnal vision*. (Text-fig. 5, 

 C, D.) 



* W. Kidd ('TliR Sense of Touch,' pp. 3-1-38, 1907) liiis tig-uved and described the 

 hands and teetof Kapalc, Saimiris, and Ccbiis froiu the point ot view of the sensory 



