TETRABELODON. 25 



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 elephant 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 true ivory is one of the most perfectly 

 elastic of substances, and is therefore specially suitable for making: 

 billiard balls. This form of ivory is not found in either the upper 

 or lower tusks of Moeritherium and Palaomastodon or in the 

 lower tusks of Dinotherium, and appears for the first time in 

 the large upper tusks of Tetrabelodon angmtidens. In some 

 of the American Tetrabelodonts in which the lower tusks are 

 very large, this structure seems to occur. 



In the cheek-teeth of Tetrabelodon angustidens (fig. 16 A) 

 great changes have also taken place. The milk-molars are still 

 replaced by premolars, but these are quickly dropped out 

 (fig. 13 B). This is chiefly the result of the great increase in 

 size of the true molars, particularly of the third, which is not 

 accompanied by a corresponding increase in the length of the 

 tooth-bearing portion of the jaws, so that there is not room for 

 the premolars and molars to remain in position at the same 

 time. The consequence of this is. that as the posterior molars 

 are cut they move forward to take up their position in the jaw, 

 thrusting out the teeth in front of them so that in the adult 

 only the two large back molars remain on each side in both 

 jaws, and in old individuals perhaps only the last is left. This 

 mode of replacement is shown in fig. 13 B. In this species, 

 while the first and second molars still have only three ridges, 

 as in Palceomastodon, the last commonly has five, and all are 

 proportionately very large. 



In the mandible (fig. 12) the anterior portion is extremely long 

 and projects much further beyond the skull than it does in Palao- 

 mastodon ; and, indeed, it is in this species that we have the 

 greatest degree of lengthening of the lower ja\v. The two lower 

 incisors, as in Palaomastodon, help to add to the length, and 

 were no doubt used for grubbing in the earth. The remarks 



