500 CHORDATA 



is possible (to a less extent in Fliascolarctos and Halinalurus). In Dasyu- 

 riis 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 aird the yourg 

 are very immature when horn. They are therefore carried a long time 1 y 

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

 posterior ventral surface, into which the nij)ples open. The ventral surface 

 is supported by the marsupial /)w;7«-, slender rods articulated, right and left, 

 at the pul)ic symphysis. Other characteristics of the marsupial skeleton 

 are the inllected posterior angle of the lower jaw (ilg. 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 aniagen, which never ile- 

 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 onlvas remnants (CffK £>/«/« and the opossums) 

 in America, but as a richly developed fauna in -\ustralia; yet no fossils are known 



Fig, 608 — Lower jaw of Thylaciniis cynoccphahis (fn m Flower), fhowint; (u) the 

 inflected angle characteristie of marsupials; cd, articular surface. 



there earlier than the pleistocene. In the latter region thev 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 ada|itation to 

 similar conditicms they have undergone a devcloi)ment analogous to that of the 

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

 carnivores, rodents, insectivores, and ungulates. 



Order I. Polyprotodonta (Zoophaga). 



INIany marsupials, among ihcm the oldest, have a dentition adapted to 

 animal food. They have numerous incisors (up to live in each upper half-iaw\ 

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

 body form, resemble the Inseclivora, others the carnivores. D.asyurid.t, 

 carnivorous: Dasyiinis; Sarcophiliis, the Tasmanian 'devil,' dangerous to larger 

 mammals; Thylachnis, pouched wolf. Per.\mklid^., insectivorous; Pcramclcs, 

 bandicoot. DiDF.LPHYrD.E, opossums, confined to America (chiefly Pouth) are 

 more carnivorous in dcntidon and recaU the apes with their opposable thumb 

 Diddphy $:■'•'■ 



