250 LLOYD’S NATURAL HISTORY. 
more nearly allied to the Australian Dasyuride, 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 Dide/phys, 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 
allied 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. 
