LIFE HABITS AND DENTITION 67 



ther feature of the dentition in the Marsupial is the fact not 

 clearly explicable at present that only one tooth at most 

 namely the last tooth preceding the molars possesses a successor. 

 This condition also occurs in the Jurassic Triconodon. 



It is probable that the reduction of the milk dentition in 

 Marsupials is a secondary specialization like the pouch forma- 

 tion and associated peculiarities relating to birth. The Miocene 

 Santa Cruz formation of Patagonia has yielded the skeletons 

 of certain undoubted Marsupials which show no fenestration 

 of the palate. In these animals the milk dentition is not so 

 greatly reduced as in modern Marsupials nor incidentally as 

 in the Jurassic Triconodon: the canines and one or two post- 

 canine teeth (milk molars) possess successors. The enamel of 

 the teeth also in the only form in which it has been microscop- 

 ically examined resembles the enamel of placental Mammals. 

 It may therefore be inferred that in diverging from their an- 

 cestral prototypes modern Marsupials have specialized in many 

 features. 



Among the existing Marsupialia are forms almost as minute 

 as the smallest Placental, others large like the Kangaroo and 

 powerful like the Tasmanian Wolf. Inhabiting only Aus- 

 tralasia and the Americas, represented indeed but sparsely in 

 the latter continent, they comprise the last remnant of a host 

 which once inhabited the entire northern hemisphere. 



The Marsupialia comprise arboreal, terrestrial and fossorial 

 animals. Some of the terrestrial forms run, others leap, a 

 few are semiaquatic. In respect of diet there are insectivorous, 

 omnivorous, carnivorous and herbivorous species. Yet with 

 all this variation in life habits the Marsupials belong to a single 

 mammalian order; all are relatively closely akin. Hence they 

 form a group in which may be traced better than in any other 

 the structural modifications of teeth associated with change of 

 habit. Marsupials as an infraclass are very ancient and the 

 precise relationships in ancestral history of the suborders exist- 

 ing today are not very clear. They are probably all derived 



