Marsh Collection.^ Pecibody Museum. 21 



or migration from this country at the close of the Bridger. 

 For this reason, I am persuaded that the evolution of the Oli- 

 gocene types took place probably in Asia or the north, during 

 later Eocene time, and that their subsequent appearance. in this 

 country during the Oligocene was due Trholly to migration. It 

 is highly probable that some generalized Wasatch species, such 

 as S. opist/iotoma of Matthew, was the ancestral type from 

 which they were derived. 



Further Observations upon the Marsupial or Metatherian 

 Relationship of the Creodonts. 



In the foregoing descriptions of the various characters of 

 the Creodonts, frequent reference has been made to their rela- 

 tions witli the Marsupials. Objections will doubtless be raised 

 to the use of this term, since some writers restrict the term 

 "Marsupial" to the living representatives of the group, which 

 is perhaps, strictly speaking, correct. Huxley j^roposed the 

 term Metatheria for a hypothetical grou]), which was meant to 

 include both the modern Marsupials and the immediate Impla- 

 cental forerunners of the Eutheria. The all-important dis- 

 tinction of such a group would consist in the implacental 

 method of its reproduction — a character which would at once 

 separate it from the Eutheria, in which an allantoic placenta 

 is formed. In like manner, the absence of a distinct coracoid, 

 the union of the odontoid process with the body of the axis, 

 the lack of oviparous habits, and the more highly developed 

 reproductive system would distinguish it from thePrototheria. 



Unfortunately there are no known characters of the skele- 

 ton in this group which are constant associates or infallible 

 correlatives of the implacental mode of reproduction, and since, 

 among the fossils, we are compelled to depend solely upon 

 osteological evidence, our judgments must of necessity rest 

 very largely on analogy. "When I have spoken of '" Marsupial 

 characters" and "Marsupial relationship," I have had con- 

 stantly in mind the Implacental Metatheria of Huxley, prefer- 

 ring the use of the term '• Marsupial," because the large number 

 of primitive characters exhibited in the living members of this 

 group undoubtedly furnish us the safest guide for a proper 

 interpretation of similar characters in the fossils. By taking 

 the more primitive members of the existing Marsupials as the 

 basis of our comparisons, I am convinced that we shall be able 

 to arrive at a very much clearer understanding of what the 

 ancestors of the Creodonta were like, than by any other method 

 of study. That they were derivatives or offshoots of any pre- 

 existing group of Placentals or Eutherians is exceedingly 

 unlikely, and the strongest evidence of this fact is that they 

 are practically as low in the scale of organization as any known 



