TETRABELODON. 23 



at the back of the skull has led to the disappearance of the Pier- 

 ridge along the middle of the roof, which is now flat. Further, 4 8 ^' 

 the tusks, which were quite small and flattened in Palaomastodon, faW 

 are now large and round, and differ from those of a modern case 23. 

 elephant only in curving downward instead of upward, and in 

 having a band of enamel along the outer side relatively much 

 narrower than in Pateomastodon, in which nearly the whole 



Fig. 12. 



Skull and mandible of Tetrabelodon anyustidens, showing the greatly elongated 

 chin with a pair of terminal cutting incisors (/./.), from the Lower and 

 Middle Miocene, France. ^V nat * s * ze ' 



war., position of opening of nose ; u.i., upper incisors. 



outer face of the tooth is enamel-clad. As the tusks increase in 

 size, the dentine of which they are composed acquires a peculiar 

 structure, which is shown in transverse sections of the tusks (see 

 table-case 24) and appears as a series of crossing lines curving 

 out from the middle of the tooth and giving a pattern like the 

 engine-turning on the case of a watch. This is quite peculiar 

 to elephant-tusks, and by it even small pieces of ivory can be 

 at once distinguished. This structure depends on the frequent 

 bending of the tubules which make up the dentine, and one result 

 of it is, that ivory is one of the most perfectly elastic of sub- 

 stances, and is therefor especially suitable for making billiard balls. 



