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 Poly- 

 protodofitia have three families in the Australian region and 

 one in America, and the same is now known to be the case 

 in the Diprotodontia. 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. Thylacoleo 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. 



