4 FIELD MUSEUM or NATURAL HISTORY ZOOLOGY, VOL. XIV. 



Skeleton Continued. 



Arm and forearm 89 



Bones of the hand 92 



Thigh and leg 93 



Bones of the foot 95 



Summary of skeletal characters 97 



Skull 99 



Summary of cranial characters 109 



Dentition in 



Number and homologies of teeth 112 



Upper incisors . . . . 116 



Lower incisors 118 



Canines 118 



Upper premolars 119 



Upper molars 120 



Lower premolars 126 



Lower molars 127 



Origin of Diprotodonty 128 



Relationships of Wynyardia 136 



Relationships of Myrmecoboides 139 



Phylogeny and taxonomy 140 



Dispersal of marsupials . 145 



General summary 150 



Literature cited 152 



The Brain of Canolestes obscurus 157 



INTRODUCTION. 



Owing to their many peculiarities of structure and their probable 

 relationship to ancient and long extinct types, marsupials are among the 

 most interesting of mammals. Their present distribution, largely in 

 Australasia but also in South America, adds to the importance they 

 possess for the morphologist and phylogenist. They have long been 

 classified in two large groups or suborders, the Polyprotodontia and the 

 Diprotodontia. These were first recognized by De Blainville in 1816 and 

 later by Owen in 1866. Broadly speaking, the polyprotodonts include 

 the carnivorous or insectivorous forms with small and relatively nu- 

 merous incisors, while the diprotodonts are the herbivorous forms 

 with the incisors reduced in number and modified much as in rodents. 

 Without exception, the diprotodonts are characterized also by a syndac- 

 tylous foot, while the polyprotodonts, with the exception of one family 

 (Peramelidae), are eleutherodactylous. This makes slightly different 

 divisions (Diadactyla and Syndactyla) possible", but other considera- 

 tions have favored the conclusion that the two major groups indicated 

 by the dentition are the most natural ones. 



Until recently, it was supposed that all modern diprotodonts and the 



