xii LIFE OF TERTIARY PERIOD 239 



during dry seasons, great droughts being prevalent in the 

 district ; but when these large animals lived there must have 

 been much more woody vegetation than there is now. 

 During the voyage of the Beagle, Darwin collected a large 

 quantity of these interesting fossils, as described in his 

 Naturalist's Voyage round the World (chap. v.). The skeleton 

 and outline figure of a Mylodon shown in Fig. 90 was 1 1 

 feet in total length, but other species were larger. 



A remarkable extinct genus, Scelidotherium, of which the 

 complete skeleton is shown in Fig. 91, was about 10 feet long, 

 and has less massive limbs than the Megatherium or Mylodon 

 and more elongated jaws. In some respects it approached 

 the ant-eaters, and was probably, like them, terrestrial in its 

 habits. About twelve distinct genera of these ground-sloths 

 are now known, comprising a large number of species. 

 They ranged all over South America and into the warmer 

 parts of North America, and before the immigration of the 

 horse and the sabre-toothed tiger in Pleistocene times, they 

 must have constituted the larger and more important portion 

 of the mammalian fauna of South America. 



Extinct Mammals of Australia 



The existing Australian mammals, although of varied form 

 and structure, are almost all marsupials, the only exceptions 

 being the aerial bats, and small rodents allied to rats, which 

 latter might have entered the country by means of floating 

 timber or trees from the nearest islands. These two orders 

 are therefore of little importance geographically, although by 

 counting the species it may be made to appear that the 

 higher mammals (Placentalia) are nearly as numerous as the 

 lower (Marsupialia). The wild dog, or dingo, is also apparently 

 indigenous, but it may have been introduced by early man, 

 as may some of the rodents. It is unfortunate that the 

 deposits of Tertiary age in Australia seem to be very scanty, 

 except recent gravels and alluvial muds, and none of these 

 have produced fossils of Mammalia except in caves and 

 dried-up lakes, which are all classed as of Pleistocene age. 

 These, however, are very productive in animal remains 

 which are extremely interesting. 



They consist of many living species, but with them 



