A kindred form, _Echidna hystrix_ or Spiny Ant-eater, is found in Australia, Tasmania, and New Guinea. The _Echidna_ hatches its young in a temporary pocket, which appears in the neighbourhood of the breasts, and disappears after the young are old enough to take care of themselves. The _Ornithorhyncus_ has fur, the _Echidna_ has spines, with hairs between them. Neither bears the slightest resemblance to a bird; the comparison suggested 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 Prototheria 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 characteristic mammals of Australasia. These are distinguished by the possession of a permanent nursery-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 attaches 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 by the muscles of the teat. [Illustration: FIG. 45.--Skull and Lower Jaw of Great Kangaroo, _Macropus giganteus_, much reduced.]