MAY, 1921. AMERICAN MARSUPIAL, QENOLESTES OSGOOD. 149 



"There is nothing unlikely in the view that they (Diprotodonta) 

 originated primarily in the North like their polyprotodont and allo- 

 therian relatives and were driven southward with the former group and 

 somewhat more thoroughly extinguished in the north, while in Aus- 

 tralia they blossomed out into a great adaptive expansion paralleling 

 the absent ungulate mammals." 



Therefore, the conclusion that the South American and Australian 

 marsupials had a common Holarctic origin is in accord with the conten- 

 tions of Matthew as to the origin of mammalian life in general. In fact 

 the case of Coenolestes might be regarded as exemplifying the manner in 

 which he has assumed that forms with irregular distribution are to be 

 explained. Thus it is not only a case in which some convergent develop- 

 ment may have occurred, but also one in which a slight addition to the 

 fossil record (i. e. Myrmecoboides) is of great importance. 



The evidence in this case may be divided as follows: 



1. The many primitive features of Canolestes indicating the 

 possible development of diprotodont characters very early 

 in the history of the order Marsupialia. 



2. The existence of very primitive characters among Australian 

 diprotodonts (see Gregory, 1910, p. 227). 



3. The large number of minor resemblances between C&nolestes 

 and the peramelids. 



4. The probable affinity of Myrmecoboides to the modern 

 peramelids. 



5. The correspondence of Myrmecoboides with the hypothetical 

 ancestor of diprotodonts, that is, the " properamelid " of 

 Bensley. 



6. The diprotodont characters shown by the earliest known 

 Australian fossil, Wynyardia. 



7. The lack of any special affinity between the caenolestids and 

 the didelphids. 



8. The probability of a more ancient origin for various mar- 

 supial groups than has been supposed (see Gidley, 1915, 

 p. 400). 



9. The fact that the Miocene caenolestids were specialized to a 

 high degree and coeval with polyprotodont types which 

 differed from them in essentially the same characters as those 

 which separate modern diprotodonts and polyprotodonts. 



10. The presumption of a northern origin of all mammalian 

 groups as indicated by Matthew. 



