MARSUPIALIA. 255 



have more than one of the teeth replaced (apparently the 

 fourth or hindermost milk-molar), while sometimes there is no 

 replacement whatever. Among the early Tertiary types just 

 mentioned, this kind of degeneration is distinctly observable ; 

 for in some genera (see p. 381) all the teeth the so-called 

 milk-teeth in front of the true molars are once shed and 

 replaced by a second set, while in the so-called Sparassodonta 

 from the Lower Tertiaries of Patagonia (p. 386), the dental 

 series closely resembles that of the Australian Dasyuridae in 

 form, and not more than two or three teeth are replaced. 

 These early Tertiary mammals, however, cannot have been 

 the direct ancestors of the Order Marsupialia they can only 

 have been persistent survivors of that ancestry ; for one known 

 mandible of Triconodon from the Upper Jurassic (Purbeck 

 Beds) of Dorsetshire exhibits the characteristic replacement of 

 the last milk-molar alone, and, unless appearances are decep- 

 tive, the loss of successional teeth in the Metatherian mammals 

 must thus have taken place at a remote time in the Mesozoic 

 period. 



ORDER 1. MARSUPIALIA. 



From the foregoing considerations it is evident that the 

 Marsupialia, as now known, are a highly specialized and modi- 

 fied order of their sub-class ; and it is unfortunate that Palaeon- 

 tology teaches nothing concerning their immediate ancestors. 

 The extinct representatives of the American types hitherto 

 discovered are scarcely different from those still surviving ; 

 while nothing is known of the forerunners of the Australian 

 types dating back further than quite the latest geological 

 period in the region they still inhabit. It is thus not surprising 

 that the division of this order into the two sub-orders of 

 POLYPROTODONTIA and DiPROTODONTiA, as usually adopted, is 

 not yet invalidated by the discovery of extinct forms. 



Sub-Order 1. Polyprotodontia. 



So far as can be judged from detached jaws, the majority of 

 the Mesozoic mammalian remains hitherto found both in Britain 

 and North America seem to be referable to this sub-order of 



