THE CLOACAL MAMMALS. 233 



Mammals was made as early as 1816 by tlie eminent 

 anatomist Blainville, who has already been mentioned, 

 and who first clearly recognised the three natural main 

 groups or sub-classes of Mammals, and distinguished them 

 according to the formation of their generative organs as 

 Omithoclelphia, Didelphia, and Monodelphia. As this 

 division is now justly considered by all scientific zoologists 

 to be the best, on account of solid foundation on the history 

 of development, let us here keep to it also. 



The first sub-class consists of the Cloaccd Animals, or 

 Breastless animals, also called Forked animals (Monotrema, 

 or Omithodelphia). This class is now represented only by 

 two species of living mammals, both of which are confined to 

 Australia and the neighbouring island of Van Diemen's land, 

 namely, the well-known Water Duck-bill (Omithorhynchus 

 paradoxus) with the beak of a bird, and the less known 

 Beaked Mole (Echidna hystrix), resembling a hedgehog. 

 Both of these curious animals, which are classed in the 

 order of Beaked Animals (Ornithostoma), are evidently the 

 last surviving remnants of an animal group formerly rich 

 in forms, which alone represented the Mammalia in the 

 secondary epoch, and out of which the second sub-class, the 

 Didelphia, developed later, probably in the Jurassic period. 

 Unfortiinately, we as yet do not know with certainty of 

 any fossil remains of this most ancient primary group 

 of Mammals, which we will call Primary Mammals (Pro- 

 mammalia). Yet they possibly comprise the oldest of all 

 the fossil Mammalia known, namely, the Microlestes antiquus, 

 of which animals, however, we as yet only know some few 

 small molar teeth. These have been found in the upper- 

 most strata of the Trias, in the Keuper, fii'st in Ger- 



