20 U.S. NATIONAL MUSEUM BULLETIN 2 73 



spider and the woolly moDkey resemble Cebus in the separation of 

 these two muscles across the face and in the absence of insertion to 

 the basis mandibulae. I regard the condition in the howler as a speciali - 

 zation. The factors responsible for such a change are perhaps the same 

 that caused the enormous enlargement of the mandibular ramus. 

 Leche (1912) correlated the modifications in the whole skull oiAlouatta 

 with the expansion of the hyoid bulla and associated structures. More 

 recently, Biegert (1963) arrived at the same conclusion. I think that 

 what affected the mandible was also operating over the whole lateral 

 aspect of the face, influencing its several structures in different degrees 

 (see muscles of the trigeminal group). The platysma colli et faciei of 

 the howler was therefore modified and underwent an expansion to the 

 point of becoming fused with the pars zygomatica of the zygomatico- 

 orbital plate. This widening may perhaps have caused also the variable 

 condition of m. triangularis in Alouatta. Sirena (1871) apparently did 

 not find this last muscle. Ruge (1887) indicated its absence in the 

 genus. Schreiber (1928) says that it is present and in a larger condition 

 than in Ateles and Lagothrix in which it seems to be a constant feature. 

 Hill (1962) declares it to be a stout structure in Brachy teles, and Cebus 

 has a large one (Ruge, 1887; Schreiber, 1928). The situation in the 

 howler with respect to this muscle could be taken to indicate that its 

 presence as a superficial structure is a morphological feature which 

 has not attained yet a fixed pattern due to the change suffered by m. 

 platysma colli et faciei. This broadening resembles to a certain degree 

 what is known about Brachy teles (Hill, 1962), a genus where the 

 mandibular ramus is also fairly large. Following Hill's description of 

 this bone in Alouatta, Brachyteles, Lagothrix, and Ateles, their mandibles 

 can be arranged in this decreasing order according to the size of the 

 ramus. The howler and the woolly spider monkey with larger platysma 

 colli et faciei are at the top. The insertion of this last muscle on the 

 mandibular basis of Alouatta (Ruge, 1887; Schreiber, 1928; this study) 

 and its absence in Ateles (Ruge, 1887; Schreiber, 1928) and Cebus 

 (Ruge, 1887; Schreiber, 1928) is perhaps an adaptation of the platysma 

 for its function of helping hold in place the large hyolaryngeal struc- 

 tures which it covers at each side of the neck. Ruge (1887) describes 

 in Lagothrix an attachment of the platysma to the basis, but Schreiber 

 (1928) says there is none. No information respecting Brachyteles is 

 available. 



Deep musculature 



A description of the styloid region in Alouatta is necessary in order 

 to have a clearer view of the muscles arising from this part of the 

 cranium. The area lies in the outer third of the posteroinferior surface 

 of the petrous bone, between the stylomastoid foramen laterally and 



