XXV] METATHERIA 661 



projects into the throat so that the air passes from the nose straight 

 down the windpipe whilst milk flows down at the sides of the air- 

 passage into the stomach. The pouch is however absent in some of 

 the more primitive Marsupials, and recent research has demon- 

 strated that it has arisen by the fusion of a number of separate 

 depressions, one of which is formed round the attachment of each 

 embryo. 



In the mandible the angle, that is to say the lower and posterior 

 end, is as a rule prolonged inwards as a horizontal shelf of bone. 

 By this feature fossil skulls are recognised as belonging to the 

 Metatheria. 



The living Metatheria are divided into two great orders, of which 

 the first is mainly carnivorous and the second herbivorous, though 

 some members of both are insectivorous. The first order is termed 



FIG. 330. Skull of Lesuenr's Kangaroo-rat, Bettongia lesueuri. To exhibit 

 Diprotodont type of dentition. 



POLYPROTODONTIA; the animals composing it have at least four incisors 

 on each side of the upper jaw and three on each side of the lower, 

 whence the name (Gr. iroXvs, many ; 717x0x09, foremost; oSou?, oSoWcs, 

 teeth). The DIPROTODONTIA, as the second order is called, derive 

 their name from the circumstance that in the lower jaw there is 

 one large pointed incisor on each side, the others being rudimentary 

 or absent, so that only two prominent teeth are observable (Gr. oYs, 

 doubly). The Polyprotodontia are represented in America by the 

 family of the Opossums, DIDELPHYIDAE, which is confined to that 

 continent. It includes 24 species, most of which are found in 

 Mexico, Central America, and Brazil, but one, the Virginian 

 Opossum, Didelphys virginiana, ranges north as far as the south 

 bank of the Hudson river. In all the Didelphyidae the great toe 

 is large and can be separated from the other toes so as with 

 them to grasp a support; thus it is said to be "prehensile." In 





