632 LAND MAMMALS IN THE WESTERN HEMISPHERE 



went back uninterruptedly to the oldest mammal-bearing 



beds of Patagonia, which may be upper Cretaceous. The 



opossums are thus the remnants of an exceedingly ancient 



group, whose beginnings are to be sought in the Mesozoic 



era and which was probably spread over all the continents. 



To all appearances, the whole group vanished completely from 



the northern hemisphere, but reentered North America from 



the south at some time during the Pliocene or early Pleistocene 



and permanently established itself here. 



The opossums are the most primitive of existing marsupials, 



especially the little South American genus, Marmosa, and are 



regarded, by some of the most competent students of the order, 



as closely representing the ancestral type of all the Recent 



families and genera, both of the Polyprotodonta and Diproto- 



donta. 



2. Thylacynidce. Predaceous Marsupials 



By many naturalists this group of flesh-eating forms is 

 included in the Dasyuridae. The family never entered North 

 America, but played a very important part in the Tertiary 

 of South America. Three existing genera of the Australian 

 region throw considerable light upon the South American 

 types, and therefore some account of them will not be out of 

 place here. 



The largest of modern predaceous marsupials is the animal 

 {Thylacynus cynocephalus) erroneously, but very naturally, 

 called the "Tasmanian Wolf," now confined to Tasmania, but 

 occurring also in the Pleistocene of Australia. As ''wolf" 

 applied to a marsupial is misleading, it will be less confusing 

 to employ the anglicized form of the generic name ''Thylacine." 

 This animal is of the size of the small Prairie Wolf or Coyote 

 (Cams latrans) and has very wolf-like appearance and habits. 

 The muzzle is long and pointed, the ears erect and rather 

 small, the tail long, very thick at the base and tapering to the 

 end, not bushy, but covered with short, close-set hairs ; the 

 colour is greyish brown, with dark, transverse stripes on the 



