6o ilULTITUBERCULAR TEETH 



A-arietv of OrdiTs. The same condition, as has been noted, char- 

 acterises that ancient Ungulate form Eiijiriilixjirnui. Even where 

 the teeth seem, at tirst sij^'ht to lie tritubercular a detailed study 

 shows traces of otherwise i,'aiiislied cusps. 



Tt must he remembered in liasing arguments upon the early 

 Jurassic and Cretaceous mammals, that our knowledge of them 

 mainly depends upon lower jaA\s, 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 siglit of. 

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

 accommodated a large series of molar teeth. Ampliitherium, for 

 example, had six inolar teeth, and five is a number freriuently 

 met with. As the teeth are so numerous and the j !ws so small 

 it seems reasonable tn 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 superabuiulant teeth 

 of many Toothed A\'hales. 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 multitubercidar type, (ir at least approach it; of this 

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

 molars of Hydrochnrrus, and of many other Eodents. 



It is well known that the fourth deciduous molar of the 

 ripper jaw, which is replaced Ijy a ]jermanent premolar in the 

 fully adidt 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 iniU<: 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 eaiiier teeth than the later-developed replacing 

 teeth. It is therefore significant that these earlier teeth shoidd 

 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 -lurassic and Cretaceous mammals. 

 It has been mentioned that absolute tritubercrdy is exceeding! v 

 rare among those ancient creatures ; more generally there are to 



