No.l.] COPE ON HORIZONS OF EXTINCT VEETEBRATA. 41 



acteristically represented than it is at tlie otlier limit of its existence, 

 viz, tlie period of its disappearance. 



For these reasons I must regard the latter criterion as the true one in 

 the discrimination of the subdivisions of geologic time, while the point 

 of the appearance of types must be looked u^ion as of provisional use 

 only, and this quite independently of the changes which discovery will 

 from time to time compel us to make in our knowledge of the distribu- 

 tion of hfe in time and space. It must, however, be borne in mind that 

 disappearance may be due to two causes : lirst, to extinction ; and, 

 secondly, to modification; a distinction which is entirely essential. 

 The case of disappearance by modification is identical with that of 

 appearance by modification, and cannot be used otherwise in classi- 

 fication. It is then the period of extinction of types to which I have 

 reference. 



With these principles in view, we continue the comparison of the ex- 

 tinct faunce of EuroiDe and I^^orth America. If we take a general view 

 of the Tertiary fauuce, we find that the following well marked types 

 representing families and higher groups have become extinct, and have 

 left no living descendants or successors : Among Insectwora^ the Leptic- 

 tidcv in IsTorth America ; also the American Bunotherian groups Tccnio- 

 donta and Tillodonta; also theilfesot^owto of both continents; ot Edentata^ 

 Macrotherium and Ancylotherium in Europe, and the Megatlieriidce in 

 North America ; among the Carnivora, the Hywnodons and Froviverrce, 

 with the Machcerodi ; of JJngulata, the entire order of Amblypoda^ 

 which, however, doubtless disappeared in some of its members by mod- 

 ification; but its only known suborders, the Pantodonta and the Dinoce- 

 rata, became absolutely extinct. Among Ferissodactyla, both continents 

 lost by extinction the Chalicotheriida^, which terminated in a great devel- 

 opment in North America ; the genera Hippotherimn* and Stylonus of 

 the line of the horses, and the Ehinocet^idce. Of Artiodactyla, two great 

 divisions, representative of each other in the two continents, totally dis- 

 appeared, viz, the Oreodontidce and the Anoplotlieriidce ; to which must 

 be added the Hyopotamidce. Of true ruminants, the most important type 

 which has disappeared from both continents is that of the Camelidce. 

 Of Suilhne genera, Anthracotherium and JElotherium may be looked upon 

 as having left no persistent successors. Last of all, the Prohoscidea 

 retreated to the continents of the south. 



In view of the complexity of the European record, I first present the 

 relations of the above mentioned phenomena as displayed in the sim- 

 13ler American system. As the present essay commences with the earliest 

 periods, I exhibit the succession in descending order on the page. The 

 horizons of the Tertiary which present distinct terrestrial faunae in 

 North America have been named the Wasatch, the Bridger, the Uinta, 

 the White Eiver, the Loup Eork, the Eqims beds, and the Ghamplain. 



* JEquus came tLrougli Protohippus, the cotemporary of Hippotherium. 



