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. _Amphitherium_, 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 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 {61} be found at least traces of more cusps. Now in some of them we may be dealing with instances of a complete tooth change; the suppression, save for one tooth, which is found in Marsupials, was probably not developed in at least some of these early mammals. The simplicity may therefore have been preceded by complexity, and may have been merely an adaptation to an insectivorous diet.