THE PLATYRRHINI. 395 



Keys. The pollex differs less from the other digits than it 

 does in the Gatarrhitii. It is more nearly parallel with, and 

 in the same plane as, the other digits of the manus; and, 

 though capable of extensive adduction and abduction, can 

 hardly be said to be truly opposable. The hallux is large, and 

 susceptible of extensive movements in abduction and adduc- 

 tion. 



The number of the dorso-lumbar vertebras varies from 

 ieventeen to twenty-two, the greatest number being pos- 

 sessed hy Nyctipithecus^ which has 33 (14 + 8 or 15+7). In 

 those forms which have prehensile tails the terminal caudal 

 vertebrae are flattened from above downward. The articular 

 surface of the head of the humerus looks more backward than 

 inward ; and, not unfrequently, there is a foramen above the 

 inner condyle. The carpus contains nine bones. The pollex 

 is generally complete, but, in Ateles, it is reduced to a small 

 metacarpal (to which, usually, a single minute nodular phalanx 

 is articulated), and is completely hidden beneath the integu- 

 ment. The pelvis is, generally, elongated, and the anterior 

 ramus of the pubis lies at right angles with the long axis of 

 the narrow ilium. The tuberosities of the ischia are everted, 

 but not rugose. In Aieles, the pelvis is broader, and the pubis 

 forms a more open angle with the ilium. The calcaneal pro- 

 cess is always very short, and compressed from side to side. 



The brain-case is rounded and devoid of strong crests. 

 There is no distinct mastoid process, and the styloid is not 

 ossified. The coronal suture is generally V-shaped, the apex 

 of the frontal bone extending far back on the vertex of the 

 skull. The alisphenoid and the parietal bones unite upon 

 the side-walls of the skull. The external auditoiy meatus is 

 not ossified, the tympanic bone retaining its foetal, hoop-like 

 form. The frontal bones approach one another on the floor of 

 the skull, but rarely unite over the junction of the presphenoid 

 with the ethmoid. On the inner surface of the periotic bone 

 there is a fossa overarched by the anterior vertical semicircular 

 canal, in which the flocculus rests. In Ateles the greater part 

 of the tentorium is ossified. In other respects, the skull pre- 

 sents extraordinary variations among the Platyrrhini ; the 

 two extremes being presented by the Howling Monkeys 

 (Mycetes) and the Squirrel Monkeys {Chrysothrix). In the 

 former, the face is very large and prominent, with a low facia] 

 angle. The roof of the brain-case is depressed ; the plane of 

 the occipital foramen is almost perpendicular to the basi-cra- 

 nial axis; and that of the tentorium is very much inclined. 



