EVOLUTION OF THE ELEPHANT LULL. 



645 



The teeth. 



It is generally true among mammals that the normal number of 

 teeth in the adult is 44, 11 in each half of 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 tooth- 

 less 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 

 number 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 re- 

 place those of the 

 milk set vertically, 

 the succession is 

 from behind for- 

 ward. The tooth 

 forms in the rear 

 of each jaw and 

 moves forward 

 through the arc of 

 a circle (see fig. 4), 

 graduallj' replac- 

 ing the preceding 

 tooth as it wears 

 away through use, 

 until the final rem- 

 nant is crowded 

 from the jaw and 

 the new tooth is in 

 full service. Bear- 

 ing 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: 



=28, which, being interpreted, means that 



Fig. 4. — Sectioned skull of Indian elephant; after Owen. 



T . 2-2 , 6 



Incisors -^ — -. molars ,- 



U — D 



■6 



there are in each half of the upper jaw two tusks, tht? 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 2 weeks and being shed 

 at the age of 2 years. The second is shed at the age of 6, the 



