II.] MONOTREMES. 33 



Although in the adult the mouth is furnished only with horny 

 plates, in young individuals the sides of the jaws are provided 

 with three pairs of molar teeth, quite unlike those of any other 

 living mammals. At the present day the duckbill is confined to 

 Queensland south of latitude 18, New South Wales, Victoria, 

 South Australia, and Tasmania; it is represented by an extinct 

 species from the Plistocene of Queensland, but otherwise the 

 pakeontological record of the group is a complete blank. 



Just the same is the case with the echidnas, or spiny anteaters 

 (Echidnidce], of which the only fossil remains known have been 

 obtained from the superficial deposits of New South Wales. 

 Terrestrial and fossorial in their habits, the echidnas differ from 

 the duckbill in having the muzzle in the form of an exceedingly 

 slender cylindrical toothless beak, furnished with an extensile 

 worm-like tongue ; while the fur is thickly mingled with short 

 spines, the tail being rudimental, and the unwebbed toes provided 

 with extremely powerful claws. Of the two species, the common 

 five-clawed echidna (Echidna aculeata] extends from south-eastern 

 New Guinea throughout Australia to Tasmania; whereas the 

 three-clawed echidna (Proechtdna 1 bruijni} is restricted to New 

 Guinea. 



With the exception of the Plistocene forms already alluded to, 

 no fossil monotremes whatever are known to science. It is, how- 

 ever, not improbable that certain extinct mammals from the 

 Secondary and lower Tertiary rocks of Arctogaea, commonly termed 

 Multituberculata, which will be more fully alluded to in the sequel, 

 may indicate a second order of the sub-class Prototheria. Both 

 the extinct and the living groups are, however, of a highly special- 

 ised type, so that the one cannot apparently be regarded as 

 ancestral to the other ; but if the presumed distant relationship 

 between the two be substantiated, it will indicate that we are to 

 look to a northern origin for the existing monotremes. 



The marsupials, which likewise represent both a separate 

 order (Marsupialia) and a sub-class (Metatheria) 



Marsupials. 



oy themselves, differ from the monotremes by 



producing living young, and by the milk-glands of the female 



1 It has recently been proposed to substitute the term Zaglossus, which is 

 stated to be earlier, for this genus. 



L. 3 



