52 THE AGE OF MAMMALS 



a clear separation of the Oligocene, Miocene, and Pliocene faunas, but we 

 cannot repress our admiration for the admirable attempts at precision on 

 the part of Hayden and Leidy whereby Oligocene and Miocene mamma- 

 lian faunas were separated ofT into six successive faunistic stages indicated 

 by the letters A, B, C, D, E, and F. 



Unfortunately this standard Avas not followed, and slow progress was 

 made for manj^ years, owing to very loose methods of collecting fossils for 

 purely anatomical and descriptive purposes without closely recording geo- 

 logic levels and life zones. Nevertheless considerable advance was made 

 in the successive writings ' of Cope (1879, 1884), Marsh (1877), Scott (1887, 

 1893), W. B. Clark (1891, 1896), Dall, (1896, 1897), Wortman (1893), Os- 

 born (1897, 1898, 1900). 



In the survey (1898) of the Middle Eocene Washakie Basin, Osborn and 

 McMaster prepared the first geologic section which recorded the 'levels' 

 on which different species of mammals were found.- The starting-point 

 of the admirable precision of recent work was Hatcher's survey between 

 1886 and 1888 of the Lower Oligocene of the Great Plains, summed up in 

 his paper, "The Titanotherium Beds," ^ in which he exactly described the 

 stratigraphy, the geographic distribution, and the division of the Titano- 

 therium Zone into Lower, Middle, and Upper levels. This was followed in 

 1893 by Wortman's paper "On the Divisions of the White River or Lower 

 Miocene of Dakota," * which treated precisely the succession of mammals 

 in the entire White River formation, now considered of Oligocene Age. 

 In 1899 all the formations both of the mountain region and of the Great 

 Plains of the West were for the first time accurately reviewed and compared 

 by Matthew in his important paper, "A Provisional Classification of the 

 Fresh- Water Tertiary of the West." * This paper was accompanied by a 

 discussion of all the preceding work of correlation, by a review of all the 

 principal formations then known, and by a complete faunal list of the spe- 

 cies of mammals hitherto described, the first which had appeared subsequent 

 to Leidy's great list published in 1869, thirty years previously. 



The next review of the American life succession during the Age of 

 Mammals is that of Osborn ('09), entitled "Cenozoic Mammal Horizons 

 of Western North America." * This comprehensive paper, accompanied 

 by "Faunal Lists of the Tertiary Mammalia of the West," by W. D. 

 Matthew, forms the American basis of the present volume. 



American and European Correlation. — This broader study has also 

 advanced step by step, beginning with the comparisons made by Leidy, 



' Principal titles are given in the Bibliography. 



* McMaster, J. B., Stratigraphical Report upon the Bridger Beds in the Washakie Basin, 

 Wyoming Territory, accompanied by profiles of three sections. In Osborn, H. F., A Memoir 

 upon Loxolophodon and Uintatherium, two Genera of the Suborder Dinocerata. Contrih. E. 

 M. Mus. Geol. Arch., College of New Jersey [Princeton], Vol. I, 1881, pp. 1-54. 



^ Hatcher, J. B., The Titanotherium Beds. Arner. Natural., March 1, 1893. pp. 204-221. 



'' For reference see Bibliography. 



