142 DK. B. A. BEXSLET ON THE EVOLUTION 



teeth show mucli the same relations. The cutting-edge tends to he duplicated behind^ 

 and a trace of this formation is also observable in Pseudochirus. 



Louder intermediate Teeth. — The only interesting feature of these teeth is the fact 

 that they are usually indicated in Pseudochirus and Petauroides, but are absent in 

 Phascolarctus, thus supporting the evidence of incisors, canines, and molars as to the 

 more advanced position of the latter genus. Even in Pseudochirus there is considerable 

 variation in the extent of development of these vestigial members as in the Phalangerina?, 

 the number varying from none to four. 



MACHOPODID^. 



Prom what has been said with reference to the preceding families, it will be apparent 

 that on passing successively through insectivorous and omnivorous modifications in the 

 Dasyuridse, Peramelidte, and the primitive Phalangeridse, we finally meet with incipient 

 herbivorous characters in the advanced members of the latter family. As will be pointed 

 out in greater detail in a subsequent section, if we follow the modifications of the foot- 

 structure in the same forms Ave find that these successive phases of dental evolution are 

 closely associated with successive phases of arboreal adaptation. The development of 

 each new phase of dental evolution in arboreal animals appears to have opened a field 

 for the continuation of that evolution in terrestrial ones. At any rate, we find that at 

 the insectivorous stage the arboreal Dasyuridse have given rise to a number of terrestrial 

 forms which now form a prominent section of the family, while at the omnivorous stage 

 another series of arboreal forms, now without typical representatives, have given rise to 

 the terrestrial Peramelidae. In the same manner the appearance of incipient herbivorous 

 characters in the Phalangeridae appears to have been marked by the origin of a series 

 of terrestrial herbivores difi'erentiating into the Macropodidae, Phascolomyidse, and 

 Diprotodontidte. 



All of the last-named fainilies are of bunodont origin — that is, if we may judge the 

 derivation of the lophodont molars of the Diprotodontidas by analogy with those of the 

 Macropodidae. Of the two principal divisions of the Phalangeridse their relations are 

 accordingly with the Phalangerinae. 



On comparing the dental charactei's of the herbivorous families it becomes apparent 

 that the phalangerine dentition is capable of giving rise to more than one line of 

 herbivorous evolution. Thus the Phascolomyidae present rodent modifications throughout 

 the dentition, while the Diprotodontidae present rodent modifications of the incisors 

 combined with lophodont characters of the molars. The Macropodidae alone show a 

 uniform continuation of that type of herbivorous evolution which is found in its incipient 

 stages in the advanced members Phalanger and Trichosurus of the Phalangerinae, althougli 

 it may be shown from the retention of the median premolars that they are the 

 descendants of forms slightly more primitive than the latter genera. 



