18 TTJSKS AND THEIR USES. 



in some detail into the structure of the tusks of the 

 elephants in the previous chapter, it will be unnecessary 

 to recapitulate the facts here ; although we may mention 

 that in modern elephants these organs consist wholly of 

 ivory, without any investing coat of enamel. The tusks 

 of these animals, which belong to the hollow type, are the 

 largest developments of dental structure to be met with 

 in the whole animal kingdom. To a great extent they are 

 weapons of defence and offence, but in the African species, 

 where they are common to both sexes, they are also largely 

 used in grubbing up roots and overturning trees ; while 

 in an extinct species from India their length is so great 

 that they must have quite ceased to be useful, and were 

 probably an actual encumbrance. Another allied extinct 

 animal, known as the dinothere, is unique in having a 

 large pair of downwardly-bent tusks in the lower jaw and 

 none in the upper ; the use of these being very difficult to 

 conjecture. 



A still more remarkable condition obtains in the 

 hippopotamus, in which not only are the canine teeth 

 developed into an enormous pair of hollow, ever-growing 

 curved tusks in each jaw, but the central pair of lower 

 front or incisor teeth are so enlarged as likewise to merit 

 the title of tusks. These incisor-tusks which thus cor- 

 respond to the lower pair of the four-tusked mastodons 

 are likewise of permanent growth, and project forwards 

 from the front of the jaw in the form of two elongated 

 cones. In thus possessing three pairs of tusks the 

 hippopotamus is quite peculiar among animals. Largely 

 employed for tearing up the grasses, on which these 

 monsters feed, the tusks of the hippopotami are also most 

 effectual offensive weapons. 



In the land carnivores, of which more anon, the tusks 

 are always of the closed type ; but in their aquatic ally, 

 the walrus, we again meet with a huge pair of ever- 

 growing tusks directed downward from the upper jaw. 

 On comparing the head of a walrus with that of an 

 elephant, most persons would say at once that the tusks 

 of the two animals were homologous. In this, however, 

 they would be wrong, since, as we learn from the con- 



