130 LLOYD'S NATURAL IIISTORY. 
Diprotodonts. Asan almost invariable rule, the molar teeth of 
the members of this division have their crowns surmounted by 
a number of sharp cusps; the general characters of the dentition 
being well shown in the accompanying figures of the skull of an 
Opossum. 
From the number and characters of their teeth, it is quite 
evident that the Polyprotodont Marsupials are more generalised 
animals than their Diprotodont allies. Evidence of this is, 
indeed, afforded by the traces of numerous vestigial incisors 
occurring in foetal Kangaroos, to which allusion has been made 
above ; and it is confirmed by the distribution of the group, 
which is the only one found beyond the confines of Australia. 
At the present day Polyprotodont Marsupials are confined to 
Australasia and America ; but during the Tertiary period the 
American Opossums were widely spread over the Old World, 
while large forms, apparently allied to the Thylacine, existed at 
the same time in South America. In still earlier epochs of the 
earth’s history, that is to say, during the Cretaceous and Juras- 
sic divisions of the great Secondary period, or the epochs in 
which our Chalk, Green-sands, and Oolites were deposited, Mar- 
supials of this group were very widely spread over the surface 
of the globe, being common in Europe and North America. 
During these epochs, and also in the preceding Triassic epoch, 
when Mammals seem to have made their first appearance on 
the earth, Polyprotodont Marsupials, together with certain im- 
perfectly known types, more or less intimately allied to the 
Monotremes, or Egg-laying Mammals, of which more anon, 
would appear to have been the dominant, if not the sole repre- 
sentatives of their class. It is also remarkable that some of | 
these extinct European Marsupials seem to have been very 
closely allied to the little Banded Anteater of Australia. From — 
the date of the Cretaceous epoch Marsupials appear to have © 
been gradually displaced from the greater portion of the earth’s — 
