562 



MARSUPIALS 



xx. 2- 



2. Classification of marsupials 



Marsupials are often divided into two suborders, the more primitive 

 insectivorous or carnivorous polyprotodonts, found outside as well as 

 within Australasia, and the diprotodonts, more specialized and re- 

 stricted. The distinction is based on the presence of more than three 



m 



Fig. 338. Hind feet of marsupials, a, opossum, with grasping hallux, arboreal; 

 B, kangaroo, without hallux, digits II and III syndactyl, cursorial; c, tree kangaroo, 

 without hallux, arboreal (secondarily). (After Bensley, from Lull, Organic Evolution, 

 copyright 1914, 1929 by The Macmillan Company, and used with their permission.) 



pairs of incisors in each jaw in the first group while in the other only 

 two remain in the lower jaw and protrude. A further interesting feature 

 is that in most diprotodonts the second and third digits of the hind- 

 limb are fused to make a comb for cleaning the hair (Fig. 338). This 

 character is absent in nearly all polyprotodonts, which are hence said 

 to be didactylous, and this is no doubt the primitive condition. How- 

 ever, the comb-like condition (syndactyly) is found not only in 

 diprotodonts but also in the polyprotodont bandicoots; conversely 

 the curious South American opossum-rat (Caetiolestes), though 

 didactylous, has diprotodont teeth. 



