JTo.l.] COPE ON HOEIZONS OF EXTINCT VEETEBEATA. 39 



between Tertiary and Cretaceous, but on the Cretaceous side of the 

 boundary, if we retain those grand divisions, which it appears to me to 

 be desirable to do. The reasons for retaining it in the Cretaceous are 

 two, viz : (1) because Dinosauria are a Mesozoic type, not known elsewhere 

 from the Tertiary 5 (2) because Mammalia (should they be found in the 

 future in the Fort Union) are not equal as e'snidence of Tertiary age, since 

 they have been also found in Jurassic and Triassic beds. The parallel- 

 ism of the American Wasatch with the TJpj)er Suessonian of France is 

 the second identification which may be regarded as provisionally estab- 

 lished. The only discordant elements at present known are the Tcenio- 

 donta of the Wasatch, which have not been so far found in Europe, and 

 the genus Lo])1iiodo7i, which is unknown in America. 



Above the Suessonian, a divergence in the characters of the European 

 and IsTorth American faunae commences, and continues to be marked 

 throughout the remainder of Tertiary time. So far as the Mammalia. 

 are concerned, the diversity between the continents was greater during 

 the periods of the Upper Eocene and Miocene than at the present era. 

 During these periods, a limited number of genera, common to the two 

 continents, was associated with numerous genera in the one which did 

 not exist in the other. As a consequence, our palseontological means of 

 identification of the horizons are limited to a restricted list, and the task 

 of applying a uniform nomenclature is, under the circumstances, diffi- 

 cult. Another difficulty in the way of determining the place of the 

 American beds in the European scale consists in the fact that the phy- 

 sical history of the two continents during the Tertiary period appears 

 to have been different. In America, the changes of level appear to 

 have been more uniform in character over large areas. Each deposit 

 has a wider geographical extent, and the fauna presents less irregular 

 variation. In Europe we have a great number of comparatively restricted 

 dejDOsits, each of which differs from the others in possessing more or less 

 peculiarity of fauna. After a study of these faunae, their natural ar- 

 rangement in Europe into three series, Eocene, Miocene, and Pliocene, 

 does not appear to rest on any solid basis. This is especially true of the 

 distinction between the first two ; and authors are at variance as to the 

 point of demarkation between the last two. Thus, the Tongrian is 

 the summit of the Eocene according to Eenevier, while Gaudry, with Fil- 

 hol and others, places it at the base of the Miocene. One oi)imon is as 

 well supported by facts, as now interpreted, as the other. 



Let us now consider the nature of the evidence on which we should 

 rely in classifying faunae and the deposits which contain them. We are 

 accustomed, at present, to rely for our definitions ujion all the faunal pe- 

 culiarities upon which we can seize : the period of appearance of certain 

 types; the duration of certain types; and the disai^pearance of certain 

 types, depending on orders, families, and genera for the major di^i.sions, 

 and species at a given locality for the lesser. It is, of course, evident 



