534 



ANATOMY OF VERTEBRATES. 



The petrosal still shows the cerebellar fossa : its exterior surface 

 is no longer swollen into a cellular bulla, but exhibits a well- 

 marked eustachian process. The ento- and post-glenoid processes 

 are well developed. The orbital border is thick and prominent, 

 but the superciliary portions do not meet above the nasal. In 

 the skull of a young Gibbon I have seen the exceptional exten- 

 sion of the frontal backward to the occipital, as in fig. 239 ( Cehus). 

 The mandibular symphysis is more nearly vertical and the angle 

 more produced in the Siamang than in other Gibbons. 



In the Orangs and Chimpanzees the foramen ovale is pierced 



in the alisphenoid, and the 

 entocarotid traverses the 

 petrosal, which has no cere- 

 bellar pit. The cranial and 

 facial parts of the skull 

 are about equal in the adult 

 males, with fully developed 

 laniary canines : in the fe- 

 males, with smaller canines, 

 the jaws are less ; and the 

 cranial cavity predominates 

 still more in the immature 

 individuals. In some va- 

 rieties of Orang [Pithecus 

 Satyi'us, fig. 355) the cra- 

 nium rises higher than in 

 others : and this feature is 

 increased, in old males, by 

 the growth of the parietal crest, Avliich bifurcates anteriorly, de- 

 fining a flat triangular space upon the frontal, and posteriorly to 

 form the lambdoid crests, — a provision, as in Carnivora, for the 

 large and powerful temporal muscles. The superorbital ridge does 

 not project above the nasal bone : this, usually single and small, is 

 flat. The premaxillaries coalesce with the maxillaries when the 

 sockets of the permanent laniaries are developed : and about the 

 same time the basisphenoid coalesces with the basioccipital. The 

 sphenoidal sinus is almost wholly formed by the presphenoid, and 

 it is divided by a longitudinal septum. The lower border of the 

 basi-occipito-sphenoidal floor of the cranium is parallel with the 

 bony palate or floor of the nostrils. The plane of the occipital 

 foramen forms an open angle with the straight basi-occipito-sphe- 

 noidal line. The alisphenoid, 6, joins the parietal, 7 ; the precondy- 

 loid foramina are usually double on each side. The mastoid is not a 



Orang {P'dhccu^ Sotijrus, male), cii 



