112 EVOLUTION OF MAMMALIAN MOLAR TEETH 



in the lower molars. The anterior shelf in the lower molars (a.s.) 

 which is prominent in the Peramelida? and Dasyuridae and very con- 

 spicuous in the molars of the Macropodidae (Fig. 58, i.\ is here absent. 

 There are also no vestiges of the external styles in the upper teeth. 



Crcscentic and Crested Herbivorous Marsupials. 



The crescentic or selenoid modification of the cusps, which is so 

 characteristic of the Artiodactyl Ungulates among Placentals, appears 

 to be developed only in Phascolarctos (Fig. 5 8 j.) among the Marsupials. 



The crested or lophoid modification of the cusps so common among 

 the perissodactyl ungulate Placentals is widely represented in the 

 Macropodidae (Fig. 58 i.}. The incipient stages leading up to this 

 crested condition are seen in the teeth of the specialized Phalanger 

 Trichosurus vulpecula (Fig. 58 k) and also in those of Hypsiprymnodon 

 moschatus among the Macropodidie, or Kangaroos. A further parallel 

 to the perissodactyl Ungulate placentals is witnessed in the develop- 

 ment of a hypselodont modification of the crested crowns among certain 

 Macropodidae. 



The derivation of all the molar types in the Marsupials from a 

 tritubercular pattern is rendered still further probable by the existence 

 of annectant forms which tend to unite the specialized, quadri-to 

 sexitubercular Diprotodonts (Kangaroos, Phalangers, Wombats), with the 

 generalized tritubercular Polyprotodonts (Dasyures and Opossums). 

 Thus Ccenolestes, the only living American Diprotodont lacks the 

 characteristic Diprotodont reduction and syndactyly of the second and 

 third digits of the hind foot and is further allied to the Polyprotodonts 

 by its close external resemblance to the Dasyurid genus Phascologale. 

 On the other hand, the Polyprotodont Bandicoots (Peramelidas) exhibit 

 the syndactyly of the Diprotodonts. Among fossil forms the gap 

 between the two sub-orders is largely bridged over by the extraordinary 

 genus Wynyardia of Baldwin Spencer,* which presents a perfect 

 melange of characters seen elsewhere only in the Opossums and 

 Daysures (Polyprotodonts), and in the Phalangers and Kangaroos 

 (Diprotodonts). 



Aberrant and Specialized Tyrjcs. 



The apparently triconodont lower molars of Tki/laci/nus are thus 

 shown to be secondary. 



The multicuspidate true molars of MyrmecoMus further support the 

 view that an elongate or basin-shaped and polybunous crown may have 

 arisen from a more tritubercular crown, because they still retain traces 



* [" A Description of Wynyardia bassiana, a Fossil Marsupial from the Tertiary Beds of 

 Table Cape, Tasmania," Proc. Zoo/. Soc. Lond., Nov. 20, 1900, pp. 776-795, PI. XLIX.] 



