374 UNGULATA 
Bay. A more nearly perfect specimen, apparently of the same species, 
was afterwards (in 1857) described under the name of Pliolophus vulpn- 
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 Hohippus. 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 
Fic. 153.—Right side of skull of Hyracotherinum leporinum, from the London Clay. }natural 
size. (After Owen.) 3, Occiput; 7, sagittal crest; 11, frontals ; 15, nasals ; 21, maxilla; 22, 
premaxilla ; d, mandibular condyle ; a, aperture of facial nerve ; 1-4, premolars ; m 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 Lophiodontide and Palwotheriide is really an arbitrary one. 
Lpihippus, 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 
’ The Swiss P. siderolithicus has only one cusp in the last upper premolar, 
