XIII 



PHYLUM CHORDATA 



555 



obliterated, if at all, in old individuals. The mental prominence 

 of the mandible is never developed, the anterior surface of the 

 symphysial region sloping backwards and downwards from the 

 bases of the incisor teeth. The stylo-hyal never gives rise to an 

 ossified styloid process. 



In the skull, as in many other respects, the Lemurs occupy an 

 intermediate position between the higher Primates and the lower 

 orders of Mammals. The occipital and ethmoidal ])lanes are 

 usually vertical. The tympanic forms a large bulla. The orbits, 

 which are large, are usually separated from the temporal fossa 

 only by a narrow rim of bone. The lacrymal foramen is situated 



on the face outside the 



margm 



of the orbit. The facial 



-jn> 



Fia. 1177.— Skull of Chimpanzee {Aathropoiilthecus tro<jlodytes). (After Blaiuvillc.) 



region is usually elongated, and may form a prominent muzzle. 

 In all the Primates the clavicle is present and complete, and 

 in the scapula the spine, acromion, and coracoid process are well 

 developed. In Man and the higher Apes the glenoid border of the 

 scapula is much longer than the coracoid border. In the lower 

 Monkeys, on the other hand, these borders are nearly equal. The 

 humerus is comparatively long and slender ; the tuberosities and 

 ridges are not, as a rule, very strongly developed. In Man and the 

 SimiidaB the bone is twisted around its long axis ; in the lower forms 

 this torsion is absent. In Man and the higher Apes the foramen 

 above the inner condyle is absent ; it is present in many of the 

 American Monkeys and in most Lemurs. Characteristic of the 

 ulna of Man and the higher Apes is the small upward extension 

 of the olecranon process. The radius and ulna are distinct in all ; 



