568 



ZOOLOGY 



SECT,. 



be recognised an absence, or a less advanced development, of some 

 of the more salient characteristics ; in other words, the earlier 

 Tertiary Mammals, when referable to existing orders, are less, 

 highly specialised than the living representatives of these orders.. 

 No less significant is the fact that these early Tertiary representa- 

 tives of existing orders had the cavity of the brain-case nearly 

 always much smaller in proportion to the other dimensions than in 

 living forms. But many are not so readily referable to existing 

 orders, sometimes owing to their possessing marked special features- 

 of their own, sometimes owing to their combining characteristic 

 features of two or more living orders. Through the series of 

 Tertiary and Post-tertiary formations it is possible to trace a gradual 

 development from the early generalised, to the existing specialised,, 

 genera, and in some instances by such gradual transitions that the 

 actual course of the evolution can be followed stage after stage. 

 There is only space here for a very brief review of this extensive 

 and remarkable Tertiary and Post-tertiary Mammalian fauna. 



No remains of Prototheria are known from the Tertiary, and it 

 is only when we come to Post-tertiary (Pleistocene) that we meet 

 with fossil representatives of the group. These, which have been 

 found only in Australia, differ little from the existing Echidna. 



Of the Marsupials the Opossums (Didelphyidse) of America are 

 represented not only in Tertiary and Pleistocene deposits in that 

 continent, but in beds of the former age in Europe. In addition, in 

 certain European deposits of Eocene age, there occur teeth and 

 jaws which may be Marsupial in character, but the affinities of 

 which are uncertain ; and in Tertiary deposits of South America 



FIG. lH,o. Diprotodon australis. (After Owen.) 



have been found numerous remains of Diprotodonts. The re- 

 mainder of the fossil Marsupials hitherto discovered are of Pleisto- 

 cene ao-e, and have all been found in Australia. The Australian 



