60 MULTITUBERCULAR TEETH CHAP. 
variety of Orders. The same condition, as has been noted, char- 
acterises that ancient Uneulate form Huprotogonia. Even where 
the teeth 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. Amphitheriuwm, 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 Polymastodon is a good example ; so, too, the 
molars of Hydrochoerus, and of many other Rodents. 
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 apphed 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 
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