IV. VERTEBRATA: MAMMALIA, MARSUP1ALIA. 633 



tooth; but it is in question whether this belongs to the second den- 

 tition or is a belated member of the first. The sexual apparatus 

 has already been described (p. 630). 



Marsupials are known from the secondary (Jurassic) and tertiary strata 

 of Europe and both Americas. They were apparently then spread over 

 the whole earth, but were crowded out by the placental mammals and 

 persisted only as remnants (Ccenolestes and the opossums) in America, but 

 as a richly developed fauna in Australia. In the latter region they con- 



FIG. 657. Lower jaw of Thylacinus cynocephalus (from Flower), showing (a) the 

 inflected angle chaiacteristic of marsupials; cd, articular surface. 



tinued because here, on account of the early separation of this continent 

 from the rest of the world, no development of Placentalia occurred. The 

 placentals are entirely lacking in Australia with the exception of those 

 introduced by man and such (mice, bats, seals) as easily pass from island 

 to island. In their present habitat, 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 half-jaw), strong canines, and sharp-pointed molars (fig. 

 657). Some in teeth, as well as in body form, resemble the Insec- 

 tivora, others the carnivores. 



The Dasyuridse are carnivorous: Dasyurus ; Sarcophilus ursinus, the 

 Tasmanian 'devil,' dangerous to larger mammals; Thytaeinus, pouched 

 wolf. The PERAMELID.E are insectivorous ; Perameles, bandicoot. The 

 DIDELPHYID^E, or opossums, which are confined to America (chiefly South) 

 are more carnivorous in dentition and recall the apes with their opposable 

 thumb. Didelphys virginiana* 



Order II. Diprotodonta (Phytophaga). 



The herbivorous habits are correlated with the degeneration 

 of canines, which usually are lacking in the lower jaw and are at 

 least very small in the upper. There are also only two incisors, 

 of large size, in the lower jaw, while the middle two of the upper 

 are much larger than the one or two lateral which may be present. 



The PHASCOLOMYID.E are the rodents of the marsupials with one chisel- 

 like incisor in each half of each ja\v. Phascalomys, wombat. The MACRO- 



