ORDER MARSUPIALIA. 1285 



known to occur in a fossil state is Pseudochirus, of which the remains 



of an existing species are found in the Pleistocene cave-deposits of 



New South Wales. The most interesting member of the family is, 



however, the large Pleistocene Thylacoleo (fig. 1156), which forms 



the type of a distinct subfamily. The dentition may be represented 



x 1 x 1 



by the formula I. -, C. -, Pm. -, M. -. The true molars and 

 1032 



earlier premolars were small, and more or less functionless, while 



the fourth premolar is enormously developed, and has a long sharp 



cutting-edge, so that, in union with its fellow of the opposing jaw, 



Fig. 1156. — Right lateral view of the skull of Thylacoleo camifex ; from the Pleistocene 

 of Australia. One-fifth natural size. 



it forms a cutting instrument of extraordinary power. In originally 

 describing this remarkable animal from fragments of jaws containing 

 the fourth premolar, Sir Richard Owen came to the conclusion that 

 the structure of this tooth indicated a carnivorous animal adapted 

 to prey upon the huge Diprotodons and Nototheres ; but the dis- 

 covery of the complete skull has shown that the animal was more 

 closely allied to the existing Phalangers, and that it could not have 

 possessed the destructive habits attributed to it by its describer, 

 although it is quite probable that its diet may have included the 

 smaller mammals, birds, and eggs. It was at one time considered 

 that the Multituberculata were allied to this genus. 



Family Macropodid^e. — The last, and in many respects the 

 most specialised family of the Diprotodontia includes the Kan- 

 garoos, Wallabies, and Kangaroo-rats. The dentition is represented 



by the formula /. -. C. -, Pm. -, M. - . The incisors are 



10 24 



extremely secant, those of the mandible being frequently movable 



against one another. The premolars may have either triangular or 



cutting crowns, and in the latter case (fig. 1158) are frequently 



grooved : the third premolar is always, and the fourth in some cases, 



