168 U.S. NATIONAL MUSEUM BULLETIN 273 



necks in order to face forward when standing or walking on their 

 four extremities. That the change in ])osition of the foramen magnum 

 is due to the modifications of the hyolaryngeal structures has been 

 clearly shown by Leche (1912) and Biegert (1963). Such an arrange- 

 ment is, nevertheless, an enormous hindrance to brachiation. The 

 fact that Alonatta rarely hangs by its arms indicates how difficult 

 it should be for this animal to move in a manner even faintly remi- 

 niscent of that practiced by the spider monkey. In spite of this 

 fact, the howler shares many details of its musculature with Ateles, 

 Brachyteles, and Lagothrix. The resemblance is striking in those 

 characters which, in at least the first two, are regarded as speciali- 

 zations to their brachiating habits. The machinery to brachiate, 

 so to speak, is there, even if the howlers do not use it but practice 

 a quadrupedal locomotion. This habit is reflected in the presence 

 of several traits indicative of pronograde stance and movements. 



I shall now discuss the regional adaptations of the musculature 

 in Alouatta. For further details the reader is referred to the sections 

 deafing with particular muscles and muscular groups. 



Adaptations in the musculature of the head. — The out- 

 standing characteristic of the muscles in the trigeminal group of 

 the howler is the i)ronounced size (figs. 1 and 4). The enlargement, 

 as compared Avith other cebids, must be related to the general process 

 of expansion that has taken place in the hyolaryngeal organs and 

 which also has affected the mandible. The mandibular insertion of 

 in. digastricus is peculiar in that it does not reach beyond the level 

 of M2 (fig. 2). It is interesting to note that in this respect there is 

 a resemblance \\ith the condition of the muscle as described by 

 Duckworth (1915) and Miirie and Mivart (1872) in lemuroids. I 

 am not yet in a position to judge whether this character in Alouatta 

 is a primitive one retained from a prosimian ancestor, or a speciali- 

 zation of the howler whereby the digastricus has apparently vnih- 

 drawn its mandibular attachment dorsally in order to leave room, 

 so to speak, for the expanding bulla hyoidea. 



The arrangement of the facial muscles seems to have advanced 

 phylogenetically to a stage comparable to that of the AteHnae, 

 but the extensive develoj)ment of the platysma colli et faciei which 

 forms a continuous muscular layer uith the orbitozygomatic plate 

 is perhaps a speciaHzation of Alouatta conditioned by the exaggerated 

 growth of the mandible (fig. 6). The variable i^resence of a triangu- 

 laris, an apparent constant feature in Ateles, Brachyteles, Lagothrix, 

 and Cebus (see Ruge, 1887; Schreiber, 1928; Hill, 1962) suggests 

 that this is a structure which has not yet attained a definite pattern, 

 probably, because of the growth influences affecting the area where 

 the muscle is located. 



