646 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



third at 9, the fourth from 20 to 25, the fifth at 60, while the 

 sixth lasts for the remainder of the creature's life, up to the age 

 of 100 to 120 years. 



The structure of a single tooth finds no exact parallel among other 

 mammals, as it consists of a series of vertically placed transverse 

 plates, each composed of a flattened mass of dentine or ivory sur- 

 rounded by a layer of enamel. The plates are in turn bound together 

 into a solid mass by a third material known as cement. "^^Tien the 

 upper surface of the tooth becomes worn through use, the hard 

 enamel appears as a series of narrow transverse ridges between which 

 lie the dentine and cement in alternate spaces, as two enamel ridges 

 with the inclosed dentine are derived from each plate. In order to 

 keep the teeth in proper condition, a certain amount of harsh, sili- 

 ceous grasses or 

 woody material is 

 necessary ; other- 

 wise the teeth be- 

 come as smooth as 

 polished marble, 

 and as the rate of 

 growth is nicely 

 adjusted to nor- 

 mal wear the 

 elephant suffers 

 greatly when giv- 

 en improper food. 

 The number of 

 plates in the larg- 

 est teeth varies 

 from 10 to 11 in 

 the African ele- 

 phant to 27 in the 

 Indian. The hairy mammoth had the most numerous and finest 

 plates of all, representing in this respect the culmination of 

 evolution. 



The tusks are merely modified incisor teeth of the upper jaw 

 which continue to grow throughout life. They are composed entirely 

 of dentine or ivory of a superlative quality, the enamel being reduced 

 to a small patch at the tip, which soon becomes worn away. The 

 tusks have various uses, but their primitive purpose is for digging. 

 The African elephant is so industrious a digger that the right tusk 

 is always the shorter, as it has to bear the brunt of the work. Tusks 

 are so small as to be apparently absent in the female Indian ele- 

 phant and often in the male, while they are present in both sexes in 



Pig. 5. — Crown view and section of a molar tooth, original. 



