OF THE AUSTRALIAN MARSUPIALIA. 185 



Culuromys. Marmoset is completely prototypal in foot-structure, apart from a slight 

 tendency in some species towards syndactylism of the second and tliird digits, the pes 

 being of a primitive arboreal type, with widely opposable hallux and well-developed, 

 striated, plantar pads. The tail is long and prehensile. The minute form M. velutissimo 

 forms a connecting-link between Mormosa and Feramt/s. 



Caliiromys is an advanced form of one of the larger species of Marmosa, such as 

 M. rapposa, M. muruia, or 31. cinerea. In its foot-structure, as also in the character 

 of its tail, it continues the arboreal developments of Marmosa. In dentition the main 

 progressive character is the omnivorous type of molar pattern in an early stage of 

 differentiation. The anterior premolars are reduced and the canines are elongate and 

 compressed. This character, at first sight appearing to indicate carnivorous habit, is in 

 all probability of secondary sexual significance. 



The peculiar form Dromiclops (Thomas, 1894) is a derivative of Marmosa, approxi- 

 mating to Caluromys in the characters of its molar teeth, and to Peramys in the 

 shortened and hairy condition of the tail. 



The two genera Metacldrus and Didelphys are so closely allied that it is not possible to 

 separate them on the characters of the dentition or foot-structure. Their dentition 

 repeats on a larger scale the characters of Marmosa. In the upper molars style Cg is 

 usually indicated on account of the greater space available on the metacone-spur. 

 Intermediate styles are poorly represented. Both genera show a slight tendency 

 towards terrestrial specialization in foot-structure. In other resj)ects Metachirus is 

 more primitive tlian Didelphys, approacliing Marmosa in its smaller size and the 

 incomplete development of the marsupium and its uniform coloration, the homogeneous 

 character of the fur, and the conformation of the ears. It is possibly related to Marmosa 

 through 31. elegans. 



CJnronecies has been pointed out by Thomas (1888) as very closely allied to 3Ietachirm, 

 and in its cranial characters indistinguishable from that genus. Apart from its obvious 

 metachirine derivation, the special features of Chironecles are remarkable and unique 

 among Marsupials, as indicating an aquatic adaptation, the tail being flattened and 

 the feet, which have a modified opposable hallux, being provided with swimming- 

 membranes connecting the digits. 



In identifying the characters in which the Didelphyidae occupy a prototypal position 

 to the Australian Marsupials, it is not possible to rely entirely on fossil forms, the 

 information available concerning them not being sufficiently full. In utilizing the 

 characters of the existing Didelphyidae to supplement the evidence of JPeralherium, the 

 facts pointed out above must be borne in mind, namely, that they represent a con- 

 temporaneous, although less extensive, radiation. Three different classes of characters 

 may thus be expected, namely : {a) those which indicate a more primitive condition than 

 is found in any of the Australian forms, as, for example, the increased lower incisor 

 formula or the arboreal type of pes with opposable hallux and non-syndactylous second 

 and third digits ; (6) those representing conditions exactly equivalent to those already 



