OF TJIE AUSTEALIAX MARSUPIALIA. 121 



by Forsyth Major that in the early Tertiary and the Cretaceous Mammalia the talonid 

 is found in a well-developed condition, does not in itself invalidate the view that tlie 

 reduced condition of this structure in the Ceutetidaj represents a primitive character, 

 tlie reverse is indicated by the fact that the modifications of the talonid in the lower 

 molars of the Insectivoi'a follow those of the paracone and metacone and also the 

 protocone in the upper. In the Talpidse, where the protocone is well developed, and 

 the paracone and metacone entirely separate from one another, the talonid is found in a 

 well-differentiated condition. In the Potamogalidte, where the paracone and metacone 

 are partially fused and the protocone slightly reduced, the talonid is also reduced ; while, 

 finally, in the Centetidte, where the fusion of the paracone and metacone is complete and 

 the protocone vestigial, the talonid is also vestigial. Such an assumption, therefore, as 

 that the changes of the lower molars in the above-mentioned Insectivora represent 

 successive stages in the development rather than the reduction of the talonid, implies 

 the further assumption of a similar sequence in the upper molars, namely the gradual 

 development of a protocone and the formation of a paracone and metacone by the 

 splitting apart of an originally single cusp, a proposition which could not be seriously 

 entertained. This conclusion, however, in nowise affects the general thesis that the 

 tuberculo-sectorial tyiie of lower molar, as exemplified by the Didelphyidoe, Dasyurida?, 

 Talpidae, and the Creodonta, has arisen by the addition of a talonid to a previous existing 

 trigonid, nor that the trituberculate condition of the upper molars of the same forms 

 represents a primitive type from which many other modifications have proceeded. Tlie 

 conditions in Notoryctes, the Chrysochloi-idce, Centetidue, and Potamogalidte simply 

 represent another line, similar to those leading to the carnivorous Dasyuridoe and tlie 

 placental Carnivora, or to the diprotodont Marsupials, the Primates and Ungulata, along 

 which the transformation of trituberculate, tuberculo-sectorial teeth may proceed. 



The dominating principle in the molar evolution of JSofori/cfes appears to be the 

 economy of space in the molar region, involving an antero-posterior compression of the 

 teeth, but the development has probably been assisted by the hypsodont modifications 

 of the molar crowns. In the upper molars of the Dasyuridae and Dideljjhyidae, tlio 

 paracone and metacone are not placed on exactly the same level as the protocone, but 

 arise together from a short median platform, to the base of which the latter cusp is 

 attached. The hypsodont development of such a crown would tend to produce just sucli 

 a condition as is seen in JS^^oluryctes, providing it Avere accomjianied by antcro-2>osterior 

 comjiression. In the Peramelidse, where there is a tendency towards an elongation 

 rather than compression of the tooth-rows, the hypsodont development is found un- 

 accompanied by a fusion of the paracone and metacone, although even in these forms 

 the latter cusps, instead of being individually elongated, are borne on an elongated 

 median pillar very similar to that seen in Notoryctes. The reduction of the longitudiniil 

 diameter of the lower molars by obliteration of the talonid calls I'or no ex])lanation. 



As already pointed out, the dentition of Notoryctes gives no indications of special 

 athnity either with that of the Dasyurid;e or that of the Peramelidoe. The only molar 

 characters on Avhich reliance may be placed in the distinction of the latter families from 

 one another relate to the presence of a hypocone in the Peramelidoe and its absence in 



