142 AMERICAN MESOZOIC MAMMALIA 



grounds in opposition to what is actually known at present. Eodelfhis and the numer- 

 ous Lance didelphids, on the contrary, while very primitive in many respects, already 

 show the fundamental special marsupial characters in the known parts as clearly as 

 does any recent member of the group. In the upper Cretaceous, marsupials and placen- 

 tal were both in existence and were clearly marked off from each other. There is no 

 actual paleontological evidence that one group is older than the other or that one group 

 was derived from the other. There is evidence, both from comparative anatomy and 

 from paleontology, that marsupials and placentals were derived from a common stock, 

 but the actual record of this divergence is wholly lacking. Presumably it is to be 

 sought in the great interval, still almost blank so far as mammals are concerned, be- 

 tween the middle or upper Jurassic and Belly River or Djadokhta time. 



It is reassuring to find that the first members of the marsupial line (and the same 

 is true of the first placentals) clearly belong to a stock which has long been adjudged 

 on other evidence to be the most primitive of its subclass. They are opossums, using the 

 term in a broad sense. At the present time the opossums, although occupying fairly 

 diverse habitats, are almost stereotyped in form. Were they known chiefly from their 

 teeth, as are their Cretaceous allies, they could hardly be placed in more than one 

 genus. They may, as Thomas has supposed, be undergoing an expansion and radiation, 

 but if so it is a radiation of limited scope or in its very first stages. In the Cretaceous, 

 on the contrary, the primitive marsupials grouped around the didelphid stem were 

 apparently in a racial phase when they were varying greatly and expanding into vari- 

 ous modes of life, that is, actively radiating. Even in our limited uppermost Cretaceous 

 collections from a single faunal facies the range in morphology and in type of adapta- 

 tion is clearly great. Such types as Tklaeodon on the one hand and Nyssodon on the 

 other represent wide extremes for a single stock not then very ancient. Along with 

 these more extreme types are found others which differ hardly at all in adaptation and 

 very little in morphology from the recent Didelfhis. From this diverse group at the 

 base of the marsupial order the increasingly specialized lines culminating in the pecul- 

 iar forms of the South American Tertiary and the Australian Pleistocene and Holo- 

 cene were probably derived. As has always been the case in the early stages of differen- 

 tiation of a mammalian stock, the majority of these potential lines of radiation were 

 undoubtedly sterile. This was certainly true in North America, where only the most 

 central and primitive phylum survived the crucial time of the close of the Cretaceous 

 period. The later opossums, Peratherium and its probable descendant Didelfhis, repre- 

 sent only the least specialized of the didelphid races which occupied this continent in 

 the later Cretaceous. 



RESUME OF ORDINAL RELATIONSHIPS 



As with the small group of the didelphids, so it has apparently always been with 

 larger groups as well. When an order, a subclass, or a class first has its origin and 

 begins its expansion it breaks up gradually into a number of diversified stocks. After 

 a time these are weeded out and only a limited number of them survive to undergo the 



