62 INTRODUCTION. 



cannot help regarding the absence of this organ (in 

 conjunction with the other points alluded to) as indi- 

 cative of an approach, in these animals, to the higher 

 orders. 



I have mentioned these, my principal reasons, for 

 placing the genus Didelphys at the head of the Mar- 

 supiata, because I regard the situation of this South 

 American group, as one of the most important dis- 

 agreements between my classification and that of 

 Professor Owen. 



Having determined to place the genus Didelphys 

 at the head of the order, my next object was to 

 arrange the remaining groups near to, or remote from 

 this typical genus, as they might approach or recede 

 from that group. 



In the Dasyuriy I found the dentition* and the 

 form of the skull almost identical with that of the 

 Opossums, and in the smaller species the similarity 

 in the external characters of these groups renders 

 it difficult to distinguish them. It might be said 

 that the Opossums have a prehensile tail, whereas 

 in the Dasyuri the tail is not prehensile; but I 

 may remark that in some of the small Opossums 

 (Didelphys brachyura, &c.) the prehensile charac- 

 ter of the tail is almost lost : for these reasons I 

 have placed the Dasyuri next the Opossums. The 

 genus Myrmecobius in most of its characters is 

 intermediate between Perameles and Phascogale ; 

 whilst Perameles appears to unite the characters of 



* The only important difference between the dentition of 

 the Dasyuridae and the Didelphida, consists in the number of 

 the incisors. 



