8 DINOCERATA. 



Above the Miocene, east of the Rock}' Mountains, and on tlie Pacific 

 coast, the PHocene is well developed, and is rich in vertebrate remains. 

 The strata rest nncoiiforniabl}' on the Miocene, and tliere is a well-marked 

 faunal change at this point, modern types now first making their 

 appearance. For these reasons, we are justified in separating the Miocene 

 from the l^liocene at this break ; although in Europe, where no great 

 break exists, the line seems to have been di-awn at a some\Ahat higher 

 horizon. Oiu' Pliocene forms essentially a continuous series, although the 

 upper beds may be distinguished from the lower l)y the presence of a 

 true Eqmis, and some other existing genera. The Pliocene climate was 

 similar to that of the Miocene. The Post-Pliocene beds contain many 

 extinct mammals, and may thus be separated from recent deposits. ^ 



With this introduction, the table of strata on jiage 7 will make clear 

 the general position of the geological horizon in w hicli the Diiiocrrata are 

 found, and especially its relation to other de})Osits of Tertiary age. To 

 make the subject clearer to the general reader, the section is enlarged to 

 incliule the whole geological series. The names ajjplied to the different 

 horizons, some used here for the first time, are, in general, those of the 

 most important vertebrates found in each, and the section thus becomes a 

 condensed index of vertebrate life in America. 



The localities in which the Dinocerafa have been found are on both 

 sides of the Green River, and mainly south of the Union Pacific Railroad, 

 in Wyoming. Of two hundred individuals in the Yale Museum, about 

 equal numbers were found east and west of this River, the distance 

 between the extreme localities in this direction being- more than one 

 hundred miles. The map on page 3 covers this region, and the more 

 important localities referred to in the volume are there indicated. 



' For a more complete presentation of this suliject, see the author's address on 

 the Introduction and tSuccession of Vertebrate Life in America, delivered before the 

 American Association for the Advancement of Science, at Nashville, Tenn., Aug., 1877. 



