402 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 48. 



origin and development of the great groups of animals which have 

 inhabited the earth, sufficient account not having been taken of the 

 great number of chapters which are as yet unsupplied by the discov- 

 eries of collectors. Thus, in the genus Phenacodus from the Wasatch, 

 Cope at one time believed he had discovered a generalized type from 

 which had originated all the Perissodactyls. It is now known, how- 

 ever, that this great group probably had a very much earlier begin- 

 ning, and it was not derived, at least, from any of the known condy- 

 larths. This is one of numerous instances in which too broad or some- 

 times obviously false generalizations have been made by investigators 

 in their search among the incomplete fossil records for ancestral forms, 

 and in their attempts at working out lines of descent. Even to-day 

 a strong tendency toward this method of reasoning seems to prevail 

 among paleontologists. It is usually assumed, in a general way, that 

 the earliest-discovered recognizable representatives of a group indi- 

 cate the actual first appearance of that group among the faunas of 

 the earth, and the absence of fossil remains of a group in the known 

 collections is usually treated as indicating its nonexistence. Such 

 assumptions, while in great part excusable perhaps, have nevertheless 

 resulted in the assigning of a much too recent date for the origin 

 and differentiation of most, if not all, of the living orders and families 

 of mammals, and doubtless have caused much of the confusion and 

 disagreement that now exists among authorities in working out cor- 

 relations and phyletic lines of descent. Many instances might be 

 given in which recent discoveries have corrected errors of this kind, 

 the tendency being to carry periods of origin further and further 

 back in time. Thus, group after group when studied in the light of 

 our increased knowlege is seen to have a much earlier beginning than 

 was assigned it a few years ago. The archaic aspect of the Paleocene 

 fauna is frequently spoken of, but such specimens as the one here 

 described, and others of similar character from the Fort Union beds, 

 make it seem probable that the very ancient appearance of the 

 known faunas of the Paleocene may be attributed in a marked degree 

 to the fact that our collections are representative of limited facies 

 and do not happen to contain many forms, doubtless living else- 

 where at the time, which if present would give a far more modern 

 aspect to the fauna of this age. The known Paleocene faunas are from 

 relatively small areas, and these of a comparatively uniform environ- 

 ment (probably in greater part forest and swamp); hence it may 

 well be that the greater number of the then existing ancestors of living 

 groups are yet unknown. 



