vi J DISTRIBUTION OF SELECTED GROUPS 115 



of earliest mammals it does not matter much where 

 they are found, because, being of at least Triassic age, 

 the whole world may have been overrun by them 

 during the enormous stretch of the Jurassic period. 

 In any case there was time for them to have reached 

 Australia, immaterial whether still quite Prototherian 

 or already recognisable as Monotremes. 



The recent Monotremes are the duck-bill, Orni- 

 thorhynchus, and a few species of spiny ant-eaters, 

 Echidna, in Tasmania and Australia, and Proechidna 

 in New Guinea. 



Marsupials. 



The pouched mammals, comprising about 1/0 

 recent species, have a very discontinuous distribution. 

 Their headquarters are now Australia with Tasmania 

 and the Papuan Islands ; two phalangers have reached 

 Celebes, and two dozen species, mostly opossums, 

 survive in America. 



It is customary to divide the marsupials into 

 I. Polyprotodonta, with three or four pairs of lower 

 incisors, e.g. Didelphidae or opossums, mostly in 

 Central and South America ; but one, Didelphys 

 virginiana, the common opossum, ranges from Argen- 

 tina far into the United States. Chironectes, the 

 little water opossum, lives in tropical America. 

 Dasyuridae comprise the 'native devils' and the wolf- 

 like Thylatinus or Tasmanian ' tiger.' Allied is the 



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