374 



UNGULATA 



Bay. A more nearly perfect specimen, apparently of the same species, 

 was afterwards (in 1857) described under the name of Pliolophus vulpi- 

 ceps, of which the skull is figured in the accompanying woodcut. 

 Other forms referable to the same genus have been obtained from 

 the Wasatch Eocene of the United States, and were described 

 by Professor Marsh under the name of Eohippus. There were four 

 premolars, the fourth being unlike the molars, and in the upper jaw 

 having only one inner cusp. The upper molars are of the general 

 type of those of Lophiodon, but have conical outer columns, and 

 the anterior transverse ridge imperfect, while the ridges of the 

 lower molars are crescentoid. Systemodon differs from Hyracotherium 



Fig. 153.— Right side of skull of Hymcotherinum leporinum, from the London Clay, i natural 

 size. (After Owen.) 3, Occiput ; 7, sagittal crest ; 11, frontals ; 15, nasals ; 21, maxilla ; 22, 

 premaxilla; d, mandibular condyle ; o, aperture of facial nerve ; p 1-4, premolars ; to 1-3, molars. 



by the absence of a diastema between the first and second pre- 

 molars ; it occurs in the Wasatch Lower Eocene of the United States. 

 In Pachynolophus (Lophiotherium, Orotherium, or Orohippus), which is 

 common to the Middle and Upper Eocene of Europe and the Bridger 

 Eocene of North America, the outer columns of the upper molars 

 are flattened, and in some cases, at least, the last premolar resembles 

 the molars, that of the upper jaw having two inner cusps. 1 This 

 genus, indeed, so closely connects Hyracotherium with the genera 

 Epihippus and Anchilophus as to show that the distinction between 

 the Lophiodontidce and Pakeotheriidce is really an arbitrary one. 

 Epihippus, of the Upper Eocene of the United States, has both the 

 third and fourth upper premolars as complex in the molars, and 

 is distinguished from Anchilophus by the lower cusps and more 

 imperfect transverse ridges of these teeth. The so-called Orohippus 

 agilis belongs to this genus. Isectolophus is another American Eocene 

 genus which may be provisionally placed in this family ; it is 

 regarded by Professors Scott and Osborn as connecting Systemodon 



1 The Swiss P. sidcrolithicus has only one cusp in the last upper premolar. 



