6o :\rULTITUBERCULAR TEETH 



variety of Orders. The same coudition, as has heeii noted, char- 

 acterises tliat ancient Ungulate form JUiip/'ofoi/onia. Even where 

 the teeth seem at first sight to he tritubercular a detailed study 

 shows traces of otherwise vanished cusps. 



It must he remembered in liasing aigumcuts 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 ])e lost sight of 

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

 accommodated a large series of molar teetli. Ampliithcrmin, for 

 example, had six inolar teeth, and live is a number fre(piently 

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

 it seems reasonal)le to connect the sim[ilicity of the structiu'e of 

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

 same argument may partly account for the superaltundant teetli 

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

 numerous grinders which are yet complex; l)ut 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 ]irovided 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 FoJipnastoihtu is a good example : so, too, the 

 molars of Hydrochoeriis, and of many other Eodents. 



It is well known that the fourth deciduous molar of tlie 

 upper jaw, wliich is replaced l)y a ])ermanent premolar in the 

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

 successor. This may indeed l)e extended to ])remolars earlier in 

 the series. In the I^og " the second and lirst milk moLirs closely 

 reseml)le the third and secontl 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 sionificant 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 tlie 

 simple teeth of some of the Jurassic and ('retaceous mammals. 

 It has been mentioned that absolute tritubercid}" is exceedingly 

 rare among those ancient creatures : more generally there are to 



