OF THE AUSTEALIAN MARSUPfALIA. 181 



Phascolomys, and Phascolarctvs, a second by the Peramelidse, and a third by the Dasj urid?e, 

 from didelphyid prototypes. Lydekker, in his ' Geographical History of Mammals,' 

 remarks as follows : — " Recent researches have tended to show that the alliance between 

 the Dasyuridse and the Didelphyidje is much more intimate than was formerly supposed 

 to be the case. This being so, it is a fairly safe assumption that both families are 

 descended from a single common ancestral stock. . . . Not improbably polyprotodont 

 Marsupials survived in south-eastern Asia till the early portion of the Eocene division of 

 the Tertiary epoch, and in this region both Dasyuridae and Didephyidae were differentiated. 

 Representatives of the former family soon afterwards found their way into Australia, 

 while the Opossums would appear to have dispersed in one direction into Europe, and in 

 the other into North America." 



Certain opinions have, however, been expressed in favour of a South- American 

 origin of the Australian fauna. Ameghino (1B91) regards the forms described by him as 

 Microbiotheriidse as ancestral to both polyprotodont and diprotodont Marsupials, as well 

 as to the Insectivora and Chiroptera. Spencer (1896) has pointed out the possibility 

 that the Australian Marsupials and the Didelphyidae may have originated at some time 

 during the Cretaceous period from South- American ancestors. In a recent paper 

 Lydekker (1899) has suggested the origin of the Dasyviridae from South- American 

 Prothylacinidse (Sparassodonta), this view being based for the most part on the resemblances 

 of Thylaciniis to that family. 



The main evidence as to the general relationships of the differeat groups of Marsupials 

 may be summarized as follows : — During the Oligocene period the Dideljiliyidii', 

 represented by Feratherium, were widely distrihuted in the northeru liemisjjherp, and if, 

 as seems very probable, the Microbiotheriidae of Ameghino, or at least some of them, arc 

 in reality members of this family, they were present at a slightly later period in South 

 America as well. Of fossil forms at present known, the Oligocene Didelphyidae are the 

 earliest which may definitely be referred to the Marsupialia. The idea that the more 

 ancient fossil Mammals must be Marsupials is untenable, and the only evidence on wliieh 

 the identification of the Jurassic forms as Marsupials now depends is the presumed 

 existence of a single tooth-change in Trlconodon, concerning which, as Lydekker (1899) 

 has pointed out, there is room for doubt. 



But whether the Oligocene Didelphyidtc were the first true Marsupials to be 

 differentiated, or whether they were the descendants of earlier Marsu])ials as yet 

 unrecognized, they are the ancestors of all of the later-appearing forms. In the develop- 

 ment of the latter there is evidence of at least three different radiations. The first and 

 most extensive one is that represented by the Australian fauna. A second is, in all 

 probability, represented by the Miocene fauna of South America. The existing 

 Didelphyidae of South America, Avhich might at first siglit be regarded as surviving 

 remnants of the original didclpliyid radiation, may be shown to rej)resent i\ third radiation 

 which is at the present time in its very incipient stages. Of these three radiations the 

 Australian and the existing South-American ones are directly traceable to minute 

 primitive didelphyid forms like the existing genera Marmosa and Feramys, or the 

 extinct Pei-athermm. 



