174 



U.S. NATIONAL MUSEUM BULLETIN 2 73 



Table 4.- — Comparison of the caudal limit of the origin of m. rhomboideus between 

 Alouatta and locomotor groups {data for semibrachiators and quadrupeds 

 taken from Ashton and Oxnard, 1963; for Alouatta, Sirena, 1871, and this 

 work) . 



Significance of difference in mean caudal limit of origin of m. rhomboideus; semibrachiators and quad 

 rupeds:p< 0.001. 



The pectoralis major of Alouatta possesses a clavicular head (fig. 

 31) (table 1). This part of the muscle is said to be small in Brachyteles 

 (Hill, 1962), but absent (Campbell, 1937) or reduced (Hill, 1962) in 

 Ateles. It is present in Lagothrix (Ashton and Oxnard, 1963; Campbell, 

 1937; Hill, 1962; Robertson, 1944). The howler resembles the semi- 

 brachiators of Ashton and Oxnard (1963) in the strong development 

 of the sternoclavicular part of the muscle, but contrasts mth their 

 quadrupeds whose sternocostal portion is the larger and extends 

 more caudally down to about the eighth costal cartilage. The pectoralis 

 minor of Alouatta arises from costal cartilages 3-5, resembling all 

 prehensile-tailed cebids except Cehus whose muscle comes from the 

 sternum (see Ashton and Oxnard, 1963; Campbell, 1937; Hill, 1960, 

 1962; Robertson, 1944). 



The rest of the forelimb muscles appear to conform to a similar 

 pattern in Alouatta, Ateles, Brachyteles, and Cehus. The pertinent 

 literature about the last four animals is cited in the sections dealing 

 with the muscles of this extremity. The subdivision of the deep 

 digital extensor seems to show great variability among these genera, 

 but this situation might be the result of incomplete observations on 

 the part of the researchers concerned, rather than true differences. 

 The separation of that muscle into a belly supplying tendons for the 

 index and poUex, and another head for the middle finger might be 

 one of the anatomical bases for the kind of manual grasp observed 

 in the howler. The extent of the radial insertion of the pronator teres 

 in Cehus, an arboreal quadruped, is shorter than in Alouatta, Ateles, 

 Brachyteles, and Lagothrix. 



Adaptations in the musculature of the lower limb. — The 

 muscular pattern of this extremity in Alouatta more closely resembles 

 that of the Atelines than that of Cehus, even if the similarities among 



