78 THE MAMMALIA OF THE DEEP RIVER BEDS. 



Width of third upper molar 004 



Length of lower molar 005 



Width of lower molar 004 



Length of third metatarsal ,019 



The specimen was found in the upper John Day beds of Deep River by Mr. C. 

 C. Jefferson. 



PERISSODACTILA. 



Equidae. 



The name Anchitherium has been very extensively applied to all those genera of 

 American equines from the different Miocene horizons which have molariform pre- 

 molars and short-crowned molars free from cement. Marsh was the first to suggest 

 the removal of the American species from this genus (No. 26, p. 248, and No. 25, 

 p. 249), and gave the names Mesoliippus and Miohippus to the species from the 

 White River and John Day beds respectively, though retaining the older term for 

 some forms from the latter group. However, the characters upon which these pro- 

 posed genera were founded, are quite insufficient for the purpose, and hence they 

 have not been widely adopted. Nevertheless, in my judgment, the separation may 

 be justified upon very different grounds and the genera established upon significant 

 structural characters. The distinction between Mesohippus, on the one hand, and 

 Miohippus, on the other, is still somewhat uncertain, though quite probable, but 

 their common differences from Anchitherium are clear. The American genera may 

 be confidently regarded as important members of the equine stem, while Anchithe- 

 rium, from present information, would appear to belong to an abortive side branch, 

 leading to no permanent results. The discovery in this country of a very large An- 

 cMtherlum of the type of the European A. aurelianense, which will be described in 

 the sequel, is somewhat unexpected and promises to be of twofold service, both in 

 determining the morphological significance and systematic position of this genus 

 and in correlating the upper Miocene horizons of the Old World and the New. The 

 uncertainty which still attends the latter question is often a most serious obstacle in 

 working out the problems of phylogeny, as well as in attempting to decipher the 

 complex history of ancient migrations and to determine in what region a given t} r pe 

 originated. 



The following table will display some of the more important differences which 

 separate the successive forms of horses occurring in the American Miocene forma- 

 tions, though there will, of course, be various opinions as to the taxonomic value 

 of these characters. 



