148 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



peramelids would be difficult to interpret from the distributional stand- 

 point were it not for the occurrence in the Eocene of North America of 

 Myrmecoboides, the imperfect remains of which are so suggestive of 

 peramelid relationship. This form seems to show that differentiation 

 along diprotodont lines was well advanced in North America during 

 the basal Eocene at an earlier period than the appearance of any of the 

 South American csenolestids, all of which are of Miocene age. Since 

 all modern peramelids are Australian while Canolestes and Myrmecoboides 

 are American, the simplest explanation of their distribution is found in 

 a common northern origin subsequent to the generalized polyprotodont 

 stage; in other words, it may be assumed that the main lines of diver- 

 gence between polyprotodonts and diprotodonts were laid down in the 

 northern hemisphere. In this case the supposed convergence or parallel- 

 ism exhibited by C&nolestes may be merely of a superficial nature, all its 

 fundamental characters having been derived directly from its northern 

 ancestor. 



The absence of fossil marsupials from Asia leaves us without direct 

 evidence of the history of the group in that region, but a wide distribu- 

 tion of the early Tertiary fauna throughout the Holarctic region is 

 generally admitted. Hence it may be assumed that if a division of the 

 marsupial stem had taken place in the early Tertiary of North America 

 the same division or one arising in a similar manner may have extended 

 to Eurasia and thence to Australia. The lack of ancient fossil types from 

 Australia is almost as complete as that from Asia. Still it is important 

 to note that Wynyardia, from the Upper Eocene or at latest Oligocene, 

 is the oldest known Australian form and has many diprotodont char- 

 acters. Moreover, the diprotodonts exhibit various primitive features, 

 in some respects (e. g. auditory ossicles) being more primitive than the 

 didelphids. They reached a very high degree of specialization in the 

 Pleistocene of Australia when such forms as Diprotodon and Thylacoleo 

 were abundant. At the present time the group is decidedly the dominant 

 one in Australia. 



The time of the introduction of marsupials into Australia has been 

 variously estimated from the Jurassic to the Eocene, recent authors 

 being inclined to the later period. Bensley concludes that it is "unlikely 

 that the marsupial radiation could have begun until well on into the 

 middle of the Tertiary period." Elsewhere (1903, p. 86) he speaks of the 

 possibility "that the ancestors of the Australian fauna passed the 

 iacipient phases of their evolution in another country, such as Asia or 

 even South America." The same idea is expressed by Matthew (1915, 

 p. 267) as follows: 



