MAMMALIA, 505 
Diprotodontia, are confined to the Australian region; but 
the selvas, occurring as they do in South America, form a 
remarkable exception. We have already seen that the Po/y- 
protodontia have three families in the Australian region and 
one in America, and the same is now known to be the case 
in the Dérotodontia. The possible explanations of this 
distribution will be given in the chapter on Geographical 
Distribution. We may here note that syndactylism, or the 
curious union of the second and third hind-toes, occurs in 
one family of the Polyprotodontia and in three families of the 
Diprotodontia, but that all these families are found in the 
Australian region. 
The Diprotodontia do not appear in the past to have had 
a much wider distribution than at present, though there are 
one or two extinct forms which are found in the same regions 
as their modern relatives. 
Diprotodon was a large rhinoceros-like animal of Pleis- 
tocene times. It is intermediate in structural characters 
between the kangaroos and the phalangers. Zhy/acoleo was 
another large phalangeroid type, and Phascolonus from 
Queensland was a large tapir-like form of wombat. These 
types would lead us to suppose that the Diprotodontia of 
Australia attained considerable dimensions in the past, 
and the absence of diprotodont remains outside the Aus- 
tralian area seems to point to an evolution of these herbivorous 
animals from polyprotodonts within that area, especially as 
the Australian remains do not date further back than the 
Pleistocene. In South America, however, the selvas and 
the fossil Epanorthus extend back to the mid-tertiary epoch, 
perhaps indicating that the diprotodont type was evolved in 
this region at an earlier period than in Australia, but was 
never so successful for want of isolation from eutherian 
types. 
