X.] PRIMATES. 141 



The periotic generally resembles that of Man, and the 

 mastoid portion is conspicuous on the outer side of the 

 skull between the squamosal and the exoccipital ; but its 

 surface is smooth and rounded, without any distinct mas- 

 toid process. 



In all the Old World species, the tympanic forms an elon- 

 gated inferior wall to the external auditory meatus, which 

 has consequently a considerable bony tube : but in all the 

 American Monkeys this bone retains more or less its primi- 

 tive annular condition, and the cavity of the tympanum is 

 close to the external wall of the cranium. This character 

 alone will readily serve to determine to which of llie two 

 great divisions of Monkeys a skull may belong. 



No auditory bulla is developed in any of the Old World 

 Monkeys, but in all the CebidcE and HapalidcE the inferior 

 surface of the ankylosed periotic and tympanic is much 

 swollen. 



The carotid canal is always very conspicuous, entering the 

 under surface of the periotic near its hinder border. There 

 is often a glenoid foramen, but never an alisphenoid canal. 



The foramen rotundum perforates the alisphenoid, but 

 the foramen ovale is usually a notch on its posterior border, 

 completed by the periotic behind. 



The mandible presents the same general characters as 

 that of Man, but the horizontal portion of the ramus 

 is usually more elongated, and the anterior border slopes 

 upwards and forwards, there being a complete absence of 

 mental protuberance. The condyle is extended trans- 

 versely, the coronoid process well developed and recurved. 

 The posterior or ascending portion of the ramus is broad 

 and flat ; the angle well developed, square, or more or 

 less rounded, but without any special pointed process as in 

 the Dog. 



