166 U.S. NATIONAL MUSEUM BULLETIN 2 73 



Nerve supply: Deep ramus of the lateral plantar nerve. 



Function: It brings the hallux into opposition with the other toes. 



Comparative anatomy of the pedal group. — The available 

 literature dealing with the pedal muscles of the platyrrhine genera we 

 are considering is scarce. HUl (1962) is the exception, as he covers 

 them all, ])articularly in the case of Brachyteles, in more detail. From 

 the information on hand, it appears as if most of these muscles in the 

 foot of all the prehensile-tailed ceboids conform to a common pattern. 

 This applies especially to the hallux and fifth-digit musculature. 



Only contrahens I and II are reported for Lagothrix, I, II, and IV 

 for Brachyteles, where they are said to correspond to the "c" pattern of 

 Jouffroy and Lessertisseur (see Hill, 1962). Ruge (1878b) studied this 

 layer in Cebiis apella and Ateles paniscus and found a full set of contra- 

 hentes in both genera; the origin of m. adductor hallucis, caput trans- 

 versum differed, however, in each genus. He described it in the spider 

 monkey as arising from (1) the distal half of the lateral margin m. 

 contrahens II, (2) the capsules of the second and third tarsometatarsal 

 joints, and (3) the medial surface of caput metatarsale II. I suspect a 

 printing error when he says, "Tarsometatarsalgelenkes" where i)erhaps 

 he meant metatarsophalangeal instead. The origin of m. adductor 

 hallucis in Cebus (Ruge, 1878b) is by two tendinous bands from the 

 capsules of the second and third metatarsophalangeal joints. The 

 oblique head in this genus cov^ers the origin of m. contrahens II, but 

 in Ateles this overlap is reduced to the proximal third of contrahens 

 II. The distribution of tendons from both heads of the flexor digitorum 

 brevis in all these animals (Glaesmer, 1910; Hill, 1962; Sawalischin, 

 1911) is highly variable. None of the i)atterns has a consistent distribu- 

 tion for a given genus. The deep head is absent in the woolly spider 

 monkey, and its interossei are said to be eight: namely, four dorsals 

 and four ventrals (Hill, 1962). 



Conclusions 



The combination of muscular characteristics that I have just de- 

 scribed in the red howling monkey is one among the series of adapta- 

 tions which this genus has developed in adjusting to a certain mode 

 of life in a given environment. That vocal communication is, and has 

 been, an important aspect of the habits of this animal is a fact sup- 

 ported by behavioral field studies (Altmann, 1959; Carpenter, 1934, 

 1960) and by the anatomical evidence of the greatly enlarged hyo- 

 laryngeal organs of the neck. 



The larynx, hyoid, and related structures attain an enormous size 

 in both sexes of adult howlers. The sounds produced by this apparatus 



