174 B. S. Lull — Evolution of the Elephant. 



each jaw. This number is rarely exceeded, but often because 

 of specialization a reduction in numbers occurs until, as in the 

 ant-bears, the limit of a totally toothless condition may be 

 reached. The elephants, owing to the great increase in the 

 size of the individual grinders and the loss of all but two upper 

 incisors in the forward part of the mouth, have the total num- 

 ber of teeth reduced apparently to six, as but one fully formed 

 grinder is in use in each half of each jaw at any one time. 



Actually, however, the number of teeth is greater than this, 

 owing to the peculiar manner of tooth succession in which, 

 instead of having the adult teeth replace those of the milk set 

 vertically, the succession is from behind forward. The tooth 

 forms in the rear of each jaw and moves forward through the 

 arc of a circle (see fig. 4.), gradually replacing the preceding 

 tooth as it wears away through use, until the final remnant is 

 crowded from the jaw and the new tooth is in full service. 

 Bearing this in mind, it is evident that the full tooth series is 

 not confined to those present at any one time, but should 

 include not only teeth which have gone before, but, in a 

 young animal, those yet to come. Sir Richard Owen gives the 

 total dentition of the modern elephant as follows : — incisors 



2 — 2 6—6 



— , molars = 28, which being interpreted means that 



there are in each half of the upper jaw two tusks, the first 

 milk tusk being succeeded by the permanent one, while in the 

 lower jaw there are none. There are all told six grinders in 

 each half of each jaw, the first appearing at the age of two 

 weeks and being shed at the age of two years. The second is 

 shed at the age of six, the third at-nine, the fourth from 

 twenty to twenty-five, the fifth at sixty, while the sixth lasts 

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

 hundred to a hundred and twenty 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 den- 

 tine or ivory surrounded by a layer of enamel. The plates are 

 in turn bound together into a solid mass by a third material 

 known as cement. When 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 

 enclosed dentine are derived from each plate. In order to 

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

 siliceous grasses or woody material is necessary, otherwise the 

 teeth become as smooth as polished marble and, as the rate of 

 growth is nicely adjusted to normal wear, the elephant suffers 



