60 MULTITUBERCULAR TEETH CHAP. 



variety of Orders. The same condition, as has been noted, char- 

 acterises that ancient Ungulate form Euprotoyonia. Even where 

 the teetli seem at first sight to be tritubercular a detailed study 

 shows traces of otherwise vanished cusps. 



It must be remembered in basing arguments upon the early 

 Jurassic and Cretaceous mammals, that our knowledge of them 

 mainly depends upon lower jaws, the teeth of which are usually 

 simpler in pattern than those of the upper jaws. Moreover, 

 another fact, not always insisted upon, must not be lost sight of. 

 In many of those creatures the jaws were of small size, and yet 

 accommodated a large series of molar teeth. AmyliitlieriuTin, for 

 example, had six molar teeth, and five is a number frequently 

 met with. As the teeth are so numerous and the jaws so small 

 it seems reasonable to connect the simplicity of the structure of 

 the teeth with the need for crowding a number together. The 

 same argument may partly account for the superabundant teeth 

 of many Toothed Whales. It is true that the Manatee has very 

 numerous grinders which are yet complex ; but then in this 

 animal there is a succession, and the jaw does not hold at a 

 given time the entire series, with which it is provided in relays. 

 On the other hand, where there are few molars they are often 

 of the multitubercular type, or at least approach it ; of this 

 the Multituberculate Polyrnastodon is a good example ; so, too, the 

 molars of Hydroclioerus, and of many other Bodents. 



It is well known that the fourth deciduous molar of the 

 upper jaw, which is replaced by a permanent premolar in the 

 fully adult animal, is of a more complex structure than its 

 successor. This may indeed be extended to premolars earlier in 

 the series. In the Dog " the second and first milk molars closely 

 resemble the third and second premolars " ; now the milk premolars 

 belong evidently to the same dentition as the permanent molars, 

 and they are earlier teeth than the later-developed replacing 

 teeth. It is therefore significant that these earlier teeth should 

 be more cuspidate than the later teeth. It tells distinctly in 

 favour of the simplification as opposed to the complication of 

 teeth in time, in the groups concerned. 



These facts may possibly be applied in explanation of the 

 simple teeth of some of the Jurassic and Cretaceous mammals. 

 It has been mentioned that absolute trituberculy is exceedingly 

 rare among those ancient creatures ; more generally there are to 



