CONFUSION IN NOMENCLATURE. 259 



In 1872 Prof. O. C. Marsh found the remains of an animal in 

 this country in deposits of Eocene age to which he applied the 

 name Orohippus* This genus was originally founded on the 

 molar teeth, which he compared with those of Anchitherium. 

 He subsequently ascertained that it possessed four toes on the 

 anterior and three on the posterior limbs. f He also proposed an- 

 other genus under the name of Eohippus, \ which he compared 

 witli Orohippus, stating that the last inferior premolar is like 

 the first true molar, a character which at once distinguishes it 

 from Hyracotherium. As he assigns no other dental characters 

 to this genus sufficient to separate it from Pliolophus, with 

 which, according to his description, it otherwise agrees, and as 

 the digital formula in the Lophiodons generally is 4 3, the 

 two names must be regarded as synonymous. This may like- 

 wise be said of the genus Orotherium ;, which Prof. Marsh 

 distinguishes by the bifid condition of the antero-internal lobe 

 of the inferior molars. This character is also ascribed to a 

 number of molar teeth discovered by Dr. Joseph Leidy in the 

 Bridger Eocene, which he referred to the genus Lophiotherium, 

 a near ally of Pliolophus. But as this is a character of very 

 doubtful generic value in this group of animals, these names 

 must be regarded as synonymous with Pliolophus. 



Assuming then that the most generalized form in the ancestry 

 of the horse hitherto known was Hyracotherium, with a digital 

 formula of 4 3 and teeth of the Lophiodon pattern, we are 

 now prepared to take a step backward to the primitive five-toed 

 ancestor, Phenacodus. But before entering on a discussion of 

 this interesting form, it is necessary to mention the discovery 

 of another genus, from the Lower Eocene beds of Wyoming, 

 which proves to be a near ally of Hyracotherium. This genus 

 Prof. Cope calls &ystemodon,\ and assigns as his reasons for 

 separating it from Hyracotherium the circumstance that it dis- 



* American Journal Science and Arts, 1872. 



t Loc. Cit., p. 247, 1874. 



% Loc. Cit., Nov., 1876. The genera Orohippus, Eohippus, Miohippus, 

 and Pliohippus have not in my estimation been distinguished from genera 

 previously described ; hence my reasons for adopting names more in 

 accordance with the prevailing nomenclature of the science. 



Loc. Cit., 1872. 



II American Naturalist, 1881, p. 1018. 



