XIII 



PHYLUM CHORDATA 



583 



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 and 

 Ornithorhynchus. 



Of the Marsupials, the Opossums (Didelphyida?) of America are 

 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 



Ss 



FIG. 1241. Diprotodon aus trails. (From a restoration of the skeleton by Prof. E. C. 

 Stirling in the Adelaide Museum.) 



which are uncertain ; and in Tertiary deposits of South America 

 have been found numerous remains of Marsupials belonging to a 

 group represented by the single surviving genus, Ccenolestes. These 

 South American Caenolestoids appear, so far as is known, to have 

 differed from the Australian Diprotodonts in the absence of the 

 characteristic syndactylism of the latter. Another group of South 

 American Tertiary Mammals, the Sparassodonts, are regarded as 

 Polyprotodonts nearly related to the Tasmanian Thylacine. The 

 remainder of the fossil Marsupials hitherto discovered are of Pleis- 

 tocene age, and have nearly all been found in Australia. The 

 Australian Pleistocene Marsupials are for the most part referable to 

 existing families and even genera, representing both the Diprotodont 

 and the Polyprotodont sections ; but some differ widely from 

 existing forms. One of these, Diprotodon (Fig. 1241), was the. 



