602 



ZOOLOGY 



SECT. 



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 the 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 

 and Ornithorhynchus. 



Of the Marsupials the Opossums (Didelphyidae) of America are 



FIG. 1229. DiprotOdon australis. (From a restoration of the skeleton by Prof. E. C. 

 Stirling in the Adelaide Museum.) 



represented not only in Cretaceous, Tertiary and Pleistocene deposits 

 in that continent, but in beds of Tertiary 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 



