352 0. C. Marsh — Recent Polydactyle Horses. 



at first supposed to have close affinities with suilline mammals. 

 Some of the latter may, in fact, be included in the species 

 referred to these genera. Various perissodactyle forms, also 

 from the Eocene of this country, have since been described by 

 Cope under the generic name Phenacodus (1873), which is 

 clearly identical with Helohyus, and some of these have been 

 referred, under the former name, to the equine ancestral line.* 

 These mammals have been placed by the writer in a distinct 

 family, the Helohyidce.^ 



Helohyidce. 



The Helohyidce may with some probability be now regarded 

 as the family from which equine mammals were derived. The 

 members of this group were small perissodactyle mammals, 

 with forty-four teeth without cement, the premolars unlike the 

 molars, and both with short bunodont crowns. The ulna and 

 fibula were complete and separate, and the feet had four or five 

 functional digits. All the known forms are from the Eocene. 

 During Tertiary time, this family apparently separated into 

 various branches, some of which became specialized, and died 

 out, while smaller forms became modified into the lines by 

 which the horse, the tapir, and the rhinoceros gradually 

 developed. 



Orohippidce. 



The successors along the first line form a well-marked family, 

 which the writer has called the Orohippidce.^ The represen- 

 tatives of this group were small equine mammals having forty- 

 four teeth without cement, incisors without pit, canine teeth 

 large, and the molar series with short crowns, and the cusps 

 more or less flattened. The ulna and fibula were complete, 

 and there were three or four functional digits in each foot. 

 The members of this family appear to be all from the Eocene 

 or Miocene. 



Equidce. 



Next in the succession came the Equidce, of which the horse 

 is a typical member. All are large equine mammals, with less 

 than forty-four functional teeth with cement, incisors with pit, 

 canines small or wanting, the molars elongated, and the pre- 

 molars essentially like the molars. The ulna and fibula are 

 incomplete, and there is but one functional toe on each foot. 

 These mammals lived in Pliocene time, continued on to the pres- 

 ent, and are now represented by the horse, ass, zebra, and quagga. 



*W. H. Flower, The Horse, LondoD, 1891; also, Madame Pavlow, L'Histoire 

 Pal. des Ongules, I-V, Moscou. 1887-1890. 



+ This Journal, vol. xiv, p. H64. November, 1877. 

 % Ibid., vol. vii, p. 249, 1874. 



