Cope.] 4')(J [May 3, 



has been admitted by Kowalewsky, Huxley, Marsh and the writer. I 

 have pointed out* that it is probable that the ancestry of some of the 

 species of Equus can be traced to Hippidium, and therefore to Protohip- 

 pus, as well as to Hippolherium, thus showing a diphyletic ancestry of 

 the true horses. Mile. Marie Pavlovf has devoted some care to the 

 attempt to demonstrate that the latter genus could not have entered into 

 this line. Her objections are derived from a consideration of the charac- 

 ters of the feet. Until however we know the structure of the feet in spe- 

 cies of Hippotherium other than tlie II. primigenium of Europe, we can- 

 not be positive as to the relation of particular species of that genus to 

 particular species of Equus. 



HIPPOTHERIUM Kaup. 



Jahrbuch fiir Mineralogie, etc., 1833, 337 (description). Nova Acta Leop. 



1835, xviii, 171-183. Jahrbuch f. Mineralogie, 1835, 623. Brpnn 



Lethsea Geognostica, 1853-6, iii, p. 876. 

 Hipparion Christol. "Annales science d, Midi, 1832, March, May," 



name only, fide Bronn. " These sur les Breches Osseuses, 1834, 26 ; 



Anniiles des Sciences Naturelles, 1835 b, v, l'J3, " name only, fide 



Bronn. 

 Anterior interior column (protocone) of the superior molars distinct 

 from the anterior interior crescent, hence giving an isolated dentinal area 

 on attrition. Second and fifth digits present on both limbs, and of reduced 

 size. Bicipital groove of humerus double. Metapodials without troch- 

 lear keel on the anterior face ; third metatarsal without entocuneiforni 

 facet. 



The preceding characters define a genus which had a general distribii 

 tion over the Northern hemisphere during late Miocene and Pliocene 

 time. It embraces a number of species, which were evidently present in 

 droves in the countries where their remains are now found. 



This genus and Protohippus include the latest representatives of the 

 three toed horses, the lateral digits being reduced to small proportions. 

 These genera represent also two types of dentition easily modifiable into 

 Equus by slight alterations in the relations of the internal columns of tl)e 

 superior molars. In Hippotherium the coalescence of the internal col- 

 umns has not been accomplished, for one of them remains in the isolated 

 position of the internal cone of the symborodont type of dentition. In 

 the species of the H. occidentale type, the anterior column is larger than 

 the posterior, displaying considerable increase in the anteroposterior diam 

 eter. This is the character of the same column in Equus, and the junc- 

 tion of the column with the adjacent crescent is all that is necessarjMo 

 convert the one genus into the other, so far as the superior molars are con- 



* American Naturalist, 1SS7, p. 1075. 



t Etudes sur I'Histoire Paltcoutologique des Ongul^s ; II. Le Development des Equidse, 

 Moscow, 188 i. 



