146 FIELD MUSEUM OP NATURAL HISTORY ZOOLOGY, VOL. XIV. 



on which his conclusions were based was vastly inferior to that now 

 available. 



Although it may be admitted that some of the Mesozoic mammals 

 were generalized marsupials, it is hardly possible to consider them in 

 efforts to follow the course of differentiation and dispersal of the modern 

 groups. For present purposes the multituberculates also may be dis- 

 regarded. From the early Tertiary we now know relatively primitive 

 types of marsupials from North America, Europe and South America. 

 Correlation of these different forms has been a matter of much difficulty 

 but according to the most recent authorities, there is little difference in 

 age between the northern and the southern types. Thus in North 

 America and Europe we have the didelphoid forms collectively placed 

 in the genus Peraikerium and regarded as of upper Eocene and Oligocene 

 age. We also have in North America Myrmecoboides, which, although a 

 less primitive form, is found in the Basal Eocene (Fort Union) . Of still 

 earlier age, but more doubtful relationship, are Pediomys and Didelphops 

 from the North American Upper Cretaceous. From South America the 

 most primitive type known is Proteodidelphys from the Notostylops 

 beds the age of which has been variously placed from Lower Cretaceous 

 to Upper Eocene. From Australia the earliest known form is Wynyardia 

 which is of at least Oligocene age and, although not primitive, has a 

 combination of diprotodont and polyprotodont characters which at this 

 early period is significant. 



Subsequent to these primitive types there are three large marsupial 

 faunas the members of which show various similarities to each other 

 and any or all of which may have been developed from the early gen- 

 eralized forms. These faunas are represented by (i) the Miocene fossils 

 from South America including the caenolestids, the borhyaenids and the 

 so-called Microbiotheridae which have didelphid characteristics; 

 (2) the present day Australian fauna with its diversity of specialization; 

 and (3) the present day American fauna all referred to the single family 

 Didelphiidae. The first and second of these faunas, broadly speaking, 

 are mutually representative, that is, the South American caenolestids 

 are at least to some degree counterparts of certain Australian diproto- 

 donts; the borhyaenids stand in the same or even closer relation to the 

 Australian thylacine; and probably it is not too much to say that the 

 Microbiotheridae, in spite of didelphid similarities, are not far removed, 

 structurally, from the generalized Australian polyprotodonts. 



This correspondence between the South American Miocene forms 

 and the Australian recent forms has not unnaturally led to the con- 

 clusion that a direct interchange between the faunas actually took place 



