1356 CLASS MAMMALIA. 



upper true molars (fig. 1228, b) resemble those of the Rhinocerot- 

 idcE ; but there appears to be a gradual transition in this respect 

 towards H. prisms, of the Quercy Phosphorites, in which these 

 teeth have rounded angles, and approximate to those of the Tapirs ; 

 from which, however, these forms are distinguished by the simpler 

 fourth premolar. This transition has induced Dr Filhol to abolish 

 the genus Protapirus, which was proposed for H. priscus. The 

 latter species is evidently allied to the American Isectolophus ; and 

 Dr Filhol suggests a transition from this type towards the Tapirs. 

 The typical American species of Hyrachyus is regarded as the 

 ancestor of the genus Hyracodon, which is classed in the Rhinocero- 

 tidce. An American species which may be provisionally included 

 in Hyrachyus is said to have an attachment for a dermal horn on 

 each nasal, on which account it is separated by Professor Marsh as 

 Colonoceros. The American Eocene genus Dilophodon appears to 

 be allied to this group. 



The imperfectly known genus Rib don, from the infra-Pampean 

 beds of Patagonia, is apparently nearly allied to Hyrachyus. The 

 genus Triplopus, of the Upper Eocene of the United States, is dis- 

 tinguished by having only three digits in the manus, on which 

 account Professor Cope makes it the type of the family Triplopod- 

 id&. In Hyracotherium, with which the forms described under 

 the names of Pliolophus and Orohippus, and not improbably Eo- 

 hippus, are identical, the upper true molars resemble those of Lophi- 

 odon, but their anterior ridge is incomplete, and the transverse ridges 

 of the lower cheek-teeth have a tendency to assume a crescent-shape. 

 In this genus there is a diastema behind the first premolar, but in 

 an allied form from the Lower Eocene of North America, which has 

 been named Systemodon, all the teeth are in contact. The numerous 

 species of Hyracotherium indicate animals not larger than a Fox ; 

 the dentition is of the full typical number, and the structure of the 

 fore-foot is shown in fig. 1236, a. This genus occurs in the Lower 

 Eocene 1 both of Europe and North America, and, as will be more 

 fully noticed below, it is regarded as the ancestral stock of the 

 Equidce. Heptodon, from the Eocene of New Mexico, which was 

 originally identified with Pachynolophus, appears to connect the 

 latter with Hyracotheriwn, although it is placed by Professor Cope 

 next to Hyrachyus. In Pachynolophus the dental formula is of the 

 full typical number ; but the upper true molars have taller ridges 

 than in Hyracotherium, and are intermediate in structure between 

 those of the latter and of Anchilophus ; the lower molars being 

 subcrescentoid. The largest species is P. isseianus, but there are 



1 It has recently been recorded from the Middle Eocene and the Phosphorites 

 of France ; but at least one of the species from the latter deposits indicates a 

 form more nearly allied to Anchilophus. 



