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 Equidce. 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 Anchitherium 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 EQUID.E. 



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 Palceotheriidce. 

 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 Equidce 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 



