EXTANT CLOACAL ANIMALS. 147 



form of the whole vertebrate class, in the Primitive 

 Mammal, by which they must have been inherited from 

 the Primitive Amnion Animals. 



During the Triassic and Jurassic Periods, the sub-class 

 of the Cloacal Animals seems to have been represented by 

 many Primitive Mammals of very varied form. At present 

 it is represented only by two isolated members, which 

 are grouped together as the Beaked Animal family (Orni- 

 tkostoma). Both of these are confined to Australia and the 

 neighbouring island of Van Diemen's Land, or Tasmania ; 

 both are becoming less numerous year by year, and will 

 soon be classed, with all their blood relations, among the 

 extinct animals of our globe. One of these forms passes 

 its life swimming about in rivers, and builds subterranean 

 dwellings on the banks: this is the well-known Duck- 

 billed Platypus (Ornithorhynchus paradoxus) : it is web- 

 footed, has a thick, soft skin, and broad, flat jaws, which 

 very much resemble a duck's bill (Figs. 195, 196). The 

 other form, the Porcupine Ant-eater (Echidna hystrix), much 

 resembles the Ant-eaters, in its mode of life, in the cha- 

 racteristic form of its slender snout, and in the great length 

 of its tongue ; it is covered with prickles, and can roll itself 

 up into a ball like a hedgehog. Neither of these extant 

 Beaked Animals possesses true bony teeth, and, in this 

 point, they resemble the Toothless Mammals (Edentata). 

 The absence of teeth, together with other peculiarities of 

 the Ornithostomata, is probably the result of comparatively 

 recent adaptation. Those extinct Cloacal Animals which 

 embraced the parent-forms of the whole Mammalian class, 

 the Promammalia, must certainly have been provided with 

 a developed set of teeth, inherited from Fishes. 157 Some 



