﻿250 ALLE^;'s naturalist's library. 



^ 



more nearly allied to the Australian Dasjun'da^, than to the 

 Opossums. Still earlier in the Tertiary period, namely in the 

 upper portion of its Eocene, or Oligocene, division there is 

 abundant evidence that Opossums, apparently inseparable from 

 the existing genus Didelphys, were spread not only over North 

 America, but likewise over a large portion of Europe. 



Passing downwards to the strata lying below the Tertiaries, 

 that is to say those equivalent to or older than the chalk, and 

 collectively forming the great Secondary system, Marsupials 

 have been found both in Europe and North America ; all of 

 which appear to belong to, or to be more or less nearly allied 

 to the Polyprotodont sub-division of the order ; the Diproto- 

 donts, as we have already had occasion to mention, being 

 quite unknown beyond the limits of the Australasian region. 

 Such primitive Secondary Marsupials occur not only in the 

 Cretaceous strata, or those equivalent to our chalk and green- 

 sands, but likewise range through the subjacent series of 

 Jurassic or Oolitic rocks, and are sparingly represented in the 

 still older beds of the Trias. 



As regards the Monotremes, we have no evidence of their 

 existence previous to the Pleistocene beds of Australia, where 

 remains referable to two of the existing genera have been dis- 

 covered. As we have already mentioned, it is, however, 

 possible that those remarkable older Tertiary and Secondary 

 Mammals known as the Multituberculata may indicate a group 

 aUied to the Monotremes, but if so, probably forming a separate 

 and distinct sub-ordinal group of the sub-class Prototheria. 



In the following brief notice of the fossil Marsupials and 

 Monotremes, all mention will be omitted of such as are identi- 

 cal with existing species ; while, from the limits of space, and 

 the imperfection of our acquaintance with the affinities of 

 many members of the former group, only the more interesting 

 and better known types will be alluded to at all. 



