PAL^EOMASTODON. 19 



structure of the skull and teeth, as well as in the rest of the Wall- 

 skeleton, so far as known, the advance is likewise very striking. case 43 - 

 In the skull (see fig. 9) the opening of the nostril (nar.) has ,^fc 

 shifted far back from near the end of the snout, though it is 

 still in front of the orbit of the eye. The nasal bones are still 



Fig. 9. 



Skull arid lower jaw of Palceomastodon, showing the elongated chin with 

 a pair of terminal incisors (/..), from the Upper Eocene (? Lower 

 Oligocene) of the Fayum, Egypt. -^ nat. size. 



nar., position of opening of nose ; u.i., upper second incisor or tusk. 



shorter and smaller than in Moeritherium. At the back of the 

 skull the development of air-cells in some of the bones has 

 enormously increased, but has not yet invaded the roof of the 

 skull, so that the sides are only separated by a sharp median 

 crest. The posterior surface of the skull slopes forward, and 

 there is a deep pit in the middle line for the attachment of the 

 muscles necessary to support the increasingly heavy head. 



Of the incisor teeth only the second pair now remains, and 

 these have been still further enlarged, forming downwardly 

 directed curved tusks with a band of enamel along their outer 

 sides only. The canine has disappeared, but there are still 

 three premolars replacing milk-molars. The anterior premolar 

 is a simple cone, while the crown of the posterior one consists 

 of two transverse ridges, these teeth being subjected to two 

 distinct influences, namely, the tendency to the reduction in the 

 front of the series, and the tendency to become more like the 



