172 U.S. NATIONAL MUSEUM BULLETIN 2 73 



Table 2. — Comparison of the cranial limit of the origin of m. serratus anterior, 

 pars caudalis between Alouatta and locomotor groups {data for semibrachiators 

 and quadrupeds taken from Ashton and Oxnard, 1963; for Alouatta, this work.) 



Significance of difference in mean cranial limit of origin of m. serratus anterior, pars caudalis; semibrachi- 

 ators and quadrupeds: p<0.001. 



It is interesting to note that the broad occipital origin of the muscle 

 occurs also in other primates where the extension of the neck acquired 

 ])articular importance in view of the presence of a large masticatory 

 apparatus, Gorilla, Papio, Como'pithecus, and Mandrillus (see Ashton 

 and Oxnard, 1963). The acromioclavicular ending of m. atlanto- 

 scapularis anterior superficial to the trapezius is another character 

 of Alouatta in common with Ateles, Brachyteles, and Lagothrix, but 

 absent in the quadrupedal Cebiis (see Ashton and Oxnard, 1963; 

 Campbell, 1937; Hill, 1960, 1962; Robertson, 1944) (table 1). 



The caudal part of the anterior serratus shows a curious mixture 

 of characters (fig. 17). Being not too thick, it resembles that of the 

 capuchin monkey (table 1). The cranial limit of its origin on rib 3 

 places the howler right at the point of overlap between the quadrupeds 

 and the semibrachiators of Ashton and Oxnard (1963), the latter 

 group including the spider and woolly monkey (table 2) ; however, 

 in the number of ribs spanned, Alouatta, with six, clearly falls mthin 

 the range of the quadrupeds (4-7) rather than among the semi- 

 brachiators (7-11) (table 3). 



In the deltoideus, which is an elevator of the arm, the pars spinalis 

 was separated from the rest of the muscle bilaterally in one male. 

 In this respect the howler slightly resembles the quadrupedal monkeys 

 (see Ashton and Oxnard, 1963). 



The rhomboideus, a stabilizer of the shoulder, and m. atlantoscapu- 

 laris posterior, an elevator of the shoulder, are enlarged in relation 

 to their possible increased significance in extending the neck (fig. 25). 

 There is no separation of the rhomboideus into two parts as in the 

 case of the semibrachiators and quadrupeds (see Ashton and Oxnard, 

 1963), and the occipital origin of the muscle is wider and thicker than 

 in any other cebid under consideration (table 1). These characters 

 conform with what would be expected if the muscle indeed acts as 



