FAMILIES OF PER1SSODACTYLA. 



701 



Cossar Ewart has recently found in the embryo of the horse the 

 rudiments of the three phalanges of the second and fourth digits. 

 The vestigial phalanges of these digits subsequently fuse with 

 one another and with the respective metacarpals or metatarsals, 

 forming "buttons" at the end of the splints. The ulna and 



3143 



fibula are incomplete. The dentition is > but the first pre- 



3 J 43 



molar is rudimentary. The orbit is completely surrounded by 

 bone. 



The modern horses are connected by a very complete series of forms 

 with ancestral Eocene types. The progress shows an increase of size, 

 a diminution in the number of digits, an increased folding of the back 

 teeth, and other differentiations. The Eocene Phenacodus is regarded 



FIG. 257. Feet of Horse and its progenitors. 

 (From NEUMAYR.) 



i. Palaeotherium ; 2. Anchitherium ; 3. Hippotherium ; 4. Equus. 



by some as near the origin of the stock, it had five complete digits on 

 each foot ; Hyracotherium and Systemodon had only four functional 

 digits in the manus ; Anchitherium from the Miocene, an animal about 

 the size of a sheep, had three digits, or three and a rudiment: 

 Hippotherium and Protohippus from the Pliocene, were as large as 

 donkeys, and show a marked diminution of the second and fourth 

 digits ; finally, in the Pleistocene, the modern forms appeared. 



The living species are the horses (Equus caballus], apparently origin- 

 ating in Asia, domesticated in prehistoric times, artificially selected into 

 many breeds, sometimes reverting to wildness as in the case of those 

 imported into America and Australia by European settlers ; the wild 

 horse of Central Asia (E. prezevalskii] ; the donkey (E. asimis] of 



