FOSSIL ANCESTORS OF THE HOKSE 



511 



Fig. H67. — Upper Molars of Fossil Ancestors of the Horse 



a, Hyracotherium ; h, Mesohippus; c, Anchitherium ; 

 (I, Protohippus; e, Hipparion; /, Horse — 1, dentine; 

 '2, enamel; 3, crusta petrosa. 



The worn or exposed surfaces of the ujDper molar teeth of five of these 

 fossil animals and of the 

 horse are represented in 

 fig. 667; in order that the 

 develoj^ment from the com- 

 paratively simple structure 

 of the tooth of the Hyra- 

 cotherium to the compli- 

 cated details of the teeth 

 of the Hipparion and horse 

 may be more readily fol- 

 lowed, those figures are 

 drawn of the same size, 

 although in nature there 

 is a gradual increase in 

 size as well as in com- 

 plexity. Of these examples 



the first three belong to the brachydont or short-ci'owned class, of which 

 a side view is given at a, fig. 668, whilst the teeth of the Protohippus 

 and Hipparion show an advance towards 

 the state of hypsodont or high -crowned 

 teeth (b, fig. 668) which culminates in 

 the horse (c, in the same figure). 



Next in chronological order to the 

 Phenacodus mention must be made of the 

 Hyracotherium and the Eohippus, also 

 from the Eocene, which are, so far as is 

 at present known, the earliest direct an- 

 cestors of the horse, the former in the Old, 

 the latter in the New World. They may, 

 indeed, be varieties of the same animal, 

 and they are described as being about 

 the size of a fox. In the fore-feet there 

 were four well -developed toes and the 

 rudiment of another, the hind-feet had 

 three toes, as represented in the Pro- 

 torohippus (fig. a, Plate LXXI), which 

 marks the next step in the order of de- 

 velopment. The change which has taken 

 place in the latter animal, as will be seen by reference to the figure, 

 consists only in the loss of the rudiment of the first digit, leaving second, 



Fig. 668. — Short- and Long-crowned 

 Molar Teeth 



a, Anchitherium; h, Hipparion; c, Horse. 



