DENTITION. SKULL. 



569 



for the posterior teeth, which are larger and have more lamellae 

 than the anterior, only make their appearance after the anterior 

 have been worn down and have fallen out. At first each half of 

 the jaw has only one grinder ; behind this a second is soon 

 developed, and so on. The whole series gradually moves for- 

 wards in the jaw and, as those in front are worn down and cast 

 off, the posterior 

 teeth are de- 

 veloped and 

 moved forwards 

 into position. 

 In Dinotherium, 

 etc., the ordin- 

 ary succession 

 is found ; the 

 milk molars are 

 vertically dis- 

 placed by pre- 

 molars and all 

 the grinding 

 teeth are in use 

 at the same 

 time. 



The principal 

 peculiarities of 

 the skull (Fig. 

 292), in addition 

 to the enormous 

 thickness of 

 some of the 

 bones owing to 

 the presence of 

 the air-spaces, 

 are the large size 



of the premaxillae which carry the tusks, the shortness 

 of the nasals, in consequence of which the bony narial 

 passage opens upwards as in whales ; the comparatively 

 slender jugal arch, the middle of which only is formed 

 by the jugal, contrary to the arrangement usually found in 

 Ungulates. The mandibles have a high vertical portion and 



FIG. 292. Skull of Ele-pkas indicus in longitudinal 

 section (from Zittel). So supraoccipital, co occ. con- 

 dyle, Pa parietal, Fr frontal, MX maxilla, Pmx pre- 

 maxilla, ME mesethmoid, ce cranial cavity, n narial 

 passage, i tusk, m l , m 2 first and second grinders. 



