8 ELEPHANTS. 



with the breaking up of the food before it is swallowed : in the 

 complete dentition there are seven of these on either side in both 

 jaws and they are divided into two groups, (1) four premolars 

 in front and (2) three molars behind: the premolars,, or at 

 least those posteriorly, replace the milk-molars of the young 

 animal, while the molars have no predecessors. It is in the 

 premolars and molars that the greatest variety of structure is 

 found, as might be expected, because it is these teeth that 

 are most affected by the nature of the food. Teeth suitable for 

 cutting up flesh would be quite unfitted for grinding hard vege- 

 table matter, and consequently in animals feeding on soft material 

 the teeth differ widely from those in which the food is hard and 

 requires much mastication. In the former the crowns of the 

 teeth are low and their cusps or tubercles are either sharp and 

 cutting in the case of flesh-feeders (e. g., lions and tigers) or 

 rounded (bunodont) in the case of animals feeding on a soft 

 vegetable or mixed diet (e.g., pigs and bears) . Teeth of this low- 

 crowned sort are called brachyodont (fig. 4, IV). In the case of 

 animals whose food is hard and requires much grinding, the 

 wear of the teeth is so great that simply-constructed low crowns 

 would be quickly worn out, and it must be remembered that 

 the length of an animal's life is largely dependent on the time 

 during which its teeth remain in good working order. To meet 

 the increased wear the crown of the tooth becomes higher and 

 moves up in the gum as wear takes place, sometimes through- 

 out the animal's life or only for a time ; teeth of this high- 

 crowned sort are called hypsodont (fig. 4, V) . With this increase 

 in height of the crown there is generally greater complication, 

 resulting from the infolding of the enamel in various ways and 

 the development of cement on the crown as well as on the roots. 

 One example of this gradual increase of height and complication 

 is about to be described in the case of the elephants ; another 

 extremely good instance is found in the gradual evolution of 

 the teeth in the horses, as excellently illustrated in the case in 

 the North Hall and in Table-case 5 (see fig. 3). 



From the above account it will be seen that when the full 

 number of teeth is present there are three incisors, one canine, 

 four premolars, and three molars on each side in the upper jaw, 



