.- )(l (, CHORD ATA 



is possible (to a less extent in Phascolarctos and Halmaturus). In Dasyu- 

 nts there is a similar connection of the yolk sac. Yet the vessels do not 

 extend with the villi into the uterine tissues as is the case with the placenta 

 of all Placentalia. In all there is insufficient nourishment and the young 

 an- \rry immature when born. They are therefore carried a long time by 

 the mother in the marsupium, a pouch formed by a fold of skin on the 

 posterior ventral surface, into which the nipples open. The ventral surface 

 is supported by the marsupial bones, slender rods articulated, right and left, 

 at the pubic symphysis. Other characteristics of the marsupial skeleton 

 are the inflected posterior angle of the lower jaw (fig. 608, a) and the rudi- 

 mentary replacement of teeth, only premolar 3 being replaced. In 

 front of the functional teeth is a row of dental anlagen, which never de- 

 velop. These are usually regarded as indicating a prelacteal dentition, 

 and the functional dentition as the milk dentition; but they may be the 

 milk dentition and the functioning teeth the permanent dentition. The 

 cloacal and sexual apparatus has already been described (p. 557). 



Marsupials occur in the Jurassic and tertiary of Europe and America. 

 They were apparently then spread over the earth, but were crowded out by the 

 placental mammals and persisted only as remnants (C&nolestes and the opossums) 

 in America, but as a richly developed fauna in Australia; yet no fossils are known 



FIG. 608 Lower jaw of Thylacinus cynocephahis (from Flower), showing (a) the 

 inflected angle characteristic of marsupials; cd, articular surface. 



there earlier than the pleistocene. In the latter region they continued because, 

 on account of the early separation of this continent from the rest of the world, 

 no development of Placentalia occurred (p. 162). In Australia, in adaptation to 

 similar conditions they have undergone a development analogous to that of the 

 placentals in other parts of the earth, so that they present groups parallel with the 

 carnivores, rodents, insectivores, and ungulates. 



Order I. Polyprotodonta (Zoophaga). 



Many marsupials, among them the oldest, have a dentition adapted to 

 animal food. They have numerous incisors (up to five in each upper half-jaw), 

 strong canines, and sharp-pointed molars (fig. 608). Some, in teeth and in 

 body form, resemble the Insectivora, others the carnivores. DASYURHX*:, 

 carnivorous: Dasyurus; Sarcophilus, the Tasmanian 'devil,' dangerous to larger 

 mammals; Thylacinus, pouched wolf. PERAMELOXE, insectivorous; Perameles, 

 bandicoot. DIDELPHYIDJE, opossums, confined to America (chiefly South) are 

 more carnivorous in dentition and recall the apes with their oppos'able thumb. 

 * 



