376 UNGULATA 
the Middle to the Upper Eocene of Europe, and usually has the full 
typical dentition, although the first premolar may disappear. The 
last lower molar has a third lobe ; and in the typical forms the last 
premolar is as complex as the molars, the diastema is short, and the 
canines are not large. In other forms, however, the hinder ridge of 
the fourth upper premolar may be aborted. The first upper cheek- 
tooth is generally a well-developed tooth, which may have a 
deciduous predecessor. Anchilophus, of the Upper Eocene of Europe, 
and Anchitherium, of the Miocene of Europe and North America, 
connect the preceding forms with the Zquide. In the latter genus 
there is the full number of teeth, the last lower molar has almost 
completely lost the third lobe of Anchilophus, and the surfaces 
of the two outer lobes of the upper molars (Figs. 157, 158) lack 
the median vertical ridges of that genus. In the American 
species of Anchitherium (which have been described as Mesohippus 
and Miohippus) the lateral digits are larger than in the European 
Middle Miocene Anchitheriwm aurelianense ; a mere splint represents 
the fifth metacarpal, and the meso- and ento-cuneiform of the tarsus 
do not unite as they do in the latter. 
Family EQuip. 
Molars hypsodont, with the outer columns of the upper ones 
flattened, the valleys completely filled with cement, and the enamel 
thrown into folds and plications ; upper premolars as complex as 
molars, which they slightly exceed in size; ridges of lower molars 
crescentoid, and complicated by enamel-foldings ; no distinct third 
lobe to last lower molar; summits of incisors with a central infold- 
ing of enamel. Orbit completely surrounded by bone. Digits 
three or one, but in the former case the median one is alone of 
functional importance ; ulna and fibula incomplete ; meso- and ento- 
cuneiform of tarsus united. 
Such are the leading characters which serve to distinguish the 
existing Horses and their nearest fossil allies from the Palwotheriide. 
The Horse, as being the best known of the Perissodactyle Ungu- 
lates, is selected for a somewhat detailed description ; but before 
proceeding to this it will be advisable to take a brief survey 
of the relations of the Hquide to the extinct forms already 
noticed, and also of the modifications of the family at present 
existing. 
The earliest form which can be certainly included in this line of 
descent is the American Lower Eocene genus Phenacodus (noticed - 
below under the head of the suborder Condylarthra), in which 
there were five complete digits to the feet. From this form there 
is but a step to Systemodon and Hyracotherium, in which the func- 
tional digits of the manus were reduced to four, as in Pachynolophus 
