90 DK. V,. A. BEXSLET ON THE EVOLUTIOX 



Cretaceous Mammalia and some of the TJngulata, liave received special designations ; 

 and three of them, as they occur in Marsupials, have been shown by Winge (1882) to 

 have definite homologies in the Placentals. Careful study, however, shows that in the 

 Marsupials no less than six of them are represented, and all of these may occur together 

 even in primitive forms (certain Didel|)hyid8e). It has therefore seemed preferable to 

 expi'ess their liomologies in the Marsupials by designating them alphabetically, rather 

 than to extend the nomenclature without a reconsideration of their number and 

 arrangement in the Placental series. The main points of cusp and style nomenclattxre 

 are illustrated in the accompanying diagram (fig. 1, p. 89). 



DASYURID^. 



Viewing the dental characters of the Australian Marsupials from an adaptive rather 

 than a taxonomic standpoint, the Dasyurida3 may be broadly distinguished from the 

 remaining members of the series by the fact that they present a trend of evolution 

 involving a progressive change from jirimitive insectivorous to specialized carnivorous 

 modifications. The position of the family in the Australian radiation is approximately 

 equivalent to that of the main insectivorous-carnivorous line represented by the 

 Creodonta, the existing Talpidse, and the Carnivora in the general placental radiation. 



All of the Australian Marsupials — and the same appears to be true of mammals 

 generally, — which are not at the present time in an insectivorous phase of dental 

 development, have passed through that phase in the course of their evolution. We 

 accordingly find in the smaller insectivorous members of the present family dental 

 modifications which, are prototypal not only to those of the more advanced carnivorous 

 forms, but also, to a considerable extent, to those of the omnivorous Peramelidse and 

 Phalangeridse, forms which are in turn prototypal in many respects to the members of 

 the herbivorous section. The smaller Dasyuridae would, no doubt, be wholly prototypal 

 in dentition, were it not for the fact that the Peramelidte present a more primitive 

 arrangement of the external cusps of the upper molars and a more primitive condition 

 of the upper incisor formula. In other words, the prototypal dental condition is nearly, 

 but not qviite, realized among the Australian Marsupials in the smaller forms of the 

 Dasyuridie. It is only in Peratherium and the existing Didelphyidae of South America 

 that all of the prototypal characters relating to the deutal evolution of the Australian 

 Marsupials may be found associated in a single form. 



The dental evolution of the Dasyuridae is not entirely homogeneous ; two of the 

 constituent genera — namely, Thijlacinus and Ilyrmecobius — may easily be shown to have 

 undergone independent development. Thijlacinus has undergone a carnivorous evolution 

 resembling so closely that of certain Neotropical forms (Sparassodonta of Ameghino), 

 while differing from that of the typical Australian members of the Dasyuridaj, as to throw 

 doubt on the propriety of its inclusion in the latter family. Mynnecob'ms has undergone a 

 special development, characterized by incipient retrogression of the dentition, as a result 

 of the adoption of the ant-eating habit. 



