OP THE AUSTEALIAX MAR.SITIALIA. 207 



radiation, which is of still more recent origin, differentiation has not proceeded beyond the 

 production of genera, rurrhermore, in some cases, even the family differentiation has 

 proceeded to such a meagre extent that family divisions are based on comparatively 

 trivial characters. In fact, the Marsupials present much the same composition as must 

 have existed at a very early stage of the placental radiation. The Australian radiation, 

 confined as it has been, has not proceeded to the stage of very great specialization or to 

 over-population, with the obliteration of less specialized intermediate types. We have 

 such cases as that of M//nneeobiiis, in which, while dental reduction has taken place as a 

 result of ant-eating habits, it has not proceeded to the stage of total obliteration of the 

 teeth, as in placental ant-eating forms. All these facts indicate that the Australian 

 radiation is of comparatively recent origin, and if, as appears probable from the central 

 position of the Creodonta, the placental radiation began in the early Eocene or late 

 Cretaceous, it seems unlikely that the marsupial radiation could have begun until well 

 on into the middle of the Tertiary period. To this structural evidence we may add the 

 final fact that the Didelphyidte are the ancestral forms of the Australian radiation, and 

 that they are typically Oligocene forms. 



The Major Classification op the Marsupials in general. 



The recognition of the Dasyurida? as a primary division in the Australian radiation 

 raises a doubt as to the applicability of Owen's classification of the Marsupials into 

 Polyprotodontia and Diprotodontia. The difficulty presented by the Petamelidtc, which 

 while essentially polyprotodont possess a syndactylous type of pes otherwise characteristic 

 of the diprotodont scries, has already been commented uj)ou by Flower, but the relations 

 of this family have been explained by Tliomas on the assimi[)tion that their syndactylism 

 has been independently acquired. 



The diprotodont modification has obviously been derived from a polyprotodont one ; it 

 is traceable to an insectivorous specialization of the median lower incisors. The members 

 of the diprotodont section may be shown to have passed through an omnivorous stage, and 

 to have possessed omnivorous molar characters similar to those of the primitive Pera- 

 nielidse. The primitive Phalangerida? are to be connected with the Peramelida^ not only 

 as syndactylous, but also as omnivorous forms, rather than separated as Diprotodontia, 

 while the Dasyuridte are to be separated as eleutherodactylous and progressively 

 carnivorou.s forms. 



In selecting a differential character on which a classification may be based wc ai'e 

 obliged to turn to other structures than the dentition. The absence of a definite hypocone 

 in the upper molars of the Dasyurida; would furnish exactly the kind of character 

 required, since the addition of this element is highly characteristic of the omnivorous 

 evolution, Avere it not for the aberrant duveloi)ments presented by Notori/ctes and 

 Thylacomys. Ttu-ning to the foot-structure, we find in the respectively eleutherodactylous 

 and syndactylous condition of the pes a difi'erential character of suflicient importance 

 and applicability. If by a major classification we iatend to designate primary lines of 



