THE VERTEBRATA l6l 



est resemblance to a bird ; the comparison sug- 

 gested in the name of Ornithorhyncus is fanciful, 

 and depends chiefly on the flat beak-like mouth; 

 these egg-laying quadrupeds may, however, be 

 reasonably brought into comparison with Reptiles. 

 Neither of them has any teeth ; the Echidna has 

 no teeth at all ; the Ornithorhyncus loses them at 

 an early stage of growth, and develops instead 

 hard horny patches in each jaw. With these it 

 crushes its food, which consists of small insects, 

 worms, etc. The Echidna, on the contrary, lives 

 in rocky places, and feeds on ants, which it 

 searches for with its long-pointed snout. These 

 two genera are grouped under the name of Proto- 

 theria or Primitive Mammals. 



The pocket in which Echidna hatches its young, 

 suggests a relationship with the next group, the 

 Metatheria or Marsupialia, which are the charac- 

 teristic mammals of Australasia. These are dis- 

 tinguished by the possession of a permanent nur- 

 sery-pocket, the "marsupium." In this they put 

 their young, which are born, like those of other 

 mammals, not hatched from eggs like those of 

 the last group. They are, however, born in a 

 very backward condition, and therefore require 

 to go through a further period of incubation, so 

 to speak, in the marsupium. Here each one at- 

 taches itself to a teat, to which it remains fixed. 

 But it cannot suck as a new-born kitten or puppy 

 does; and the milk is forced down its throat bjl 

 the muscles of the teat. 



The Marsupialia are not entirely confined to 

 Australasia; a few occur in South America, and 

 in North America they are represented by the 

 " 'possum," /. e. Opossum, of American stories. 

 The Marsupials seem almost to mimic the forms 



