MAMMALIA 493 



very modest role, and are nearly all of minute size. In America 

 they have been found only in the fresh-water beds at the base and 

 summit of the Cretaceous, and in Europe only in the Purbeck and 

 Wealden at the base. The Lower Cretaceous mammals differ 

 little from those of the Jura (except for the larger number of 

 genera), and from the fragmentary condition of the specimens it 

 is exceedingly difficult to determine just what groups are repre- 

 sented. The Multituberculata are believed to belong to the 

 lowest type of mammals, the Monotremata, at present represented 

 only by the Spiny Ant-eater {Echidna) and Duck-billed Mole 

 ( Ornithorhynclius) of Australia. Of this group the most promi- 

 nent Lower Cretaceous genera are the English Plagiaulax and the 

 American Ctenacodon and Allodon. In another group the teeth 

 are much simpler but more numerous ; examples are Stylodon and 

 Triconodon from the English Purbeck, and Dryokstes and Dicro- 

 cynodon from Wyoming. In the uppermost Cretaceous the mam- 

 mals are much more numerous and diversified, and already 

 begin to show affinities with the forms which are to succeed them 

 in the Tertiary. The Multituberculata are represented by two 

 genera, Meniscoessus and Ptilodus, while other mammals of doubt- 

 ful affinities are Didelphops, Pediomys, and Cimolestes. Many 

 others are known, but they are too imperfect for reference. With 

 one exception, Thlczodon, which is of moderate size, all these 

 mammals are exceedingly small. 



In brief, Cretaceous life is still typically Mesozoic, but a change 

 toward Cenozoic conditions is already manifest, especially in the 

 Plants, the Gastropods, and the Teleostean Fishes. There is still 

 a gap between the life systems of the two eras, but it is not so 

 wide as it was once believed to be, and it may be hoped that 

 future discoveries will bridge it entirely. 



