SUCCESSIVE MAMMALIAN FAUNAS 251 



seem to be all comprised in a single genus {'fProtomeryx) which 

 was the same as that found in the lower Miocene. A very 

 small and dainty little creature {^Hypertragulus) belonged to 

 another family, the relationships of which are not clear. 



To the middle and lower Oligocene is referred the great 

 White River formation of South Dakota, Nebraska, Wyoming, 

 etc., which is divisible into three clearly marked substages. 

 The White River contains the best-known fauna of all of the 

 North American Tertiaries, for collecting in these beds has 

 been carried on for more than sixty years, and a greater number 

 of complete and nearly complete skeletons has been secured 

 than from any of the other formations. It is plainly evident 

 that a land-connection existed with the Old World, which was 

 interrupted in the John Day, as is shown by the intermigration 

 of characteristic forms ; but some barrier, presumably climatic, 

 prevented any complete interchange of mammals, and very 

 many genera and even famiUes remained confined to one con- 

 tinent or the other. 



The aspect of the White River fauna changes in accordance 

 with the direction from which it is approached. If one comes 

 to the study of it from the Eocene, it displays a very modern 

 aspect, given by the almost complete disappearance of the 

 archaic groups of mammals and by the great multipUcation 

 of genera and species belonging to the progressive orders. 

 These genera, it is true, are all extinct, but many of them stood 

 in an ancestral relationship to modern forms. On the other 

 hand, if approached from the Miocene side, the White River 

 mammals seem to be very ancient and primitive and very 

 different from anything that now Uves. We speak of horses 

 and rhinoceroses, dogs and cats, in this fauna, but those terms 

 can be employed only in a very wide and elastic sense to desig- 

 nate animals more or less distantly allied to those of the present 

 day. 



Several species of opossums, some of them very small, were 

 the only marsupials in North America then, as they are now. 



