THE AUSTRALIAN REGION. 341 



of Australia is imperfect, as all yet discovered are from 

 Post-Tertiary or very late Tertiary desposits. It is 

 interesting to find, however, that all belong to the 

 marsupial type, although several are quite unlike any 

 living animals, and some are of enormous size, almost 

 rivalling the mastodons and megatheriums of the northern 

 continents. In the earliest Tertiary formation of Europe 

 remains of marsupials have been found, but they all 

 belong to the opossum type, which is unknown in 

 Australia ; and this supports the view that no commu- 

 nication existed between the Paleearctic and Australian 

 regions even at this early period. Much farther back, 

 however, in the Oolite and Trias formations, remains of 

 a number of small mammalia have been found which 

 are almost certainly marsupial, and bear a very close 

 resemblance to the Myrmecobius, a small and very rare 

 mammal still living in Australia. An animal of some- 

 what similar type has been discovered in rocks of the 

 same age in North America ; and we have, therefore, 

 every reason to believe, that it was at or near this 

 remote epoch when Australia, or some land which has 

 been since in connection with it, received a stock of 

 mammalian immigrants from the great northern conti- 

 nent ; since which time it has almost certainly remained 

 completely isolated. 



The occurrence of the marsupial opossums in America 

 has been thought by some writers to imply an early 

 connection between that continent and Australia ; but 

 the fact that opossums existed in Europe in Eocene and 

 Miocene times, and that no trace of them has been found in 

 North or South America before the Post-Pliocene period, 

 renders it almost certain that they entered America 



