328 THE ANATOMY OF VERTEBRATED ANIMALS. 



chorion ; and a small allantois ; and to be devoid of a thymus 

 gland. 



The Dideljihia are at present confined to the Australian 

 and the Austro-Columbian provinces, some few species 

 stretching beyond the borders of the latter into the northern 

 parts of North America. The BidelpliidiB alone are found 

 in Austro-Columbia, all the other groups being Australian. 



Gigantic, Kangaroo-like, or Phalangistic, forms {Notothe- 

 rirnn, Diprotodon, Thylacoleo), have been found in post-tertiary 

 deposits and caves in Australia. In Europe, DidelphidcB 

 occur in Eocene strata; Didelpliidce, Dasyuridce, and Ma- 

 cropodidee {Phascolotherium, Amphitherium, Flagiaulax), in 

 middle Mesozoic rocks ; and Macropodidoe (?) {Microlestes) in 

 the Trias. 



III. The MoNODELPHiA.^In the Monodelphia, the os 

 odontoidemn very soon becomes ankylosed with the second 

 cei-vical vertebra, of which it appears merely as the odontoid 

 process ; and the cei-vical ribs early become inseparably 

 united with theij" vertebrae. The coracoid is reduced to a 

 mere process of the scapula, and there is no epicoracoid 

 similar to that of the Ornithodelphia. 



Clavicles may be present or absent. When completely 

 developed they articiilate directly, or by the intermediation 

 of more or less modified remains of the sternal end of the 

 coracoid, with the sternum, and not with any interclavicle. 

 The acetabula are imperforate. The pelvis is devoid of 

 marsupial bones ; though, in some Carnivora, there are small 

 cartilages in the inner tendons of the external oblique 

 muscle, which have a corresponding form and relations. 



The anterior commissure and the corpus callosum, no less 

 than the cerebral hemispheres themselves, vary greatly, the 

 brains of some Edentata very closely approaching those of 

 the Didelpliia in respect of the corpus callosum and an- 

 terior commissure; while, as regards the hemispheres 

 themselves, they may either be so small as to allow the cere- 

 bellum to be completely exposed on the dorsal aspect, or so 

 large, as completely to cover it and project beyond it. The 



