350 THE VERTEBRATE SKELETON. 



There is no precoracoid (epicoracoid) or interclavicle, and the 

 coracoid is reduced to form a mere process of the scapula, not 

 coming near the sternum. 



Epipubic, or so-called marsupial bones 1 , nearly always 

 occur, and a fourth pelvic element, the acetabular bone, is 

 frequently developed. The fibula is always complete at its 

 distal end, sometimes it is fused with the tibia, but often it is 

 not only free but is capable of a rotatory movement on the 

 tibia. This is the case in the families Phascolomyidae, Didel- 

 phyidae, and Phalangeridae. 



The Marsupialia can be subdivided into two main groups, 

 according to the character of the teeth : 



1. POLYPROTODONTIA. 



In this group the incisors are small, subequal and nume- 

 rous, not less than . The canines are larger than the 

 incisors, and the molars have sharp cusps. The members of 

 this group are all more or less carnivorous or insectivorous. 

 The group includes the families Didelphyidae, Dasyuridae, 

 Peramelidae, and Notary ctidae a . 



2. DlPROTODONTIA. 



In this group the incisors do not exceed , and are usually 

 f , occasionally i. The first upper and lower incisors are large 

 and cutting. The lower canines are always small or absent, 

 and so in most cases are the upper canines. The molars have 

 bluntly tuberculated, or transversely ridged crowns. The 

 group includes the families Phascolomyidae, Phalangeridae, 

 Macropodidae, and Epanorthidae. 



1 These bones however have no connection with the marsupium, 

 being nearly equally developed in both male and female. They are 

 simply sesamoid bones forming ossifications in the. inner tendon of the 

 external oblique muscle, and are developed as supports for the abdominal 

 wall. Very similar structures have been independently developed in 

 various Amphibians, Reptiles and monodelphian Mammals. See W. Leche, 

 Biol. Foren. in. p. 120. 



2 See H. Gadow, P. Z. S. 1892, p. 361. 



