FAMILIES OF PERISSODACTYLA. 581 



metacarpals and metatarsals of the second and fourth. The ulna 



and fibula are incomplete. The dentition is ftH , but the first 



premolar 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 



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 ; Hipparioti 



and Protokippus 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 those imported into 

 America and Australia by European settlers ; the wild horse of Central 

 Asia (E. prezevalskii) ; the donkey (E. asinus) of African origin, the 

 wild asses of Africa and Asia, the striped African species — the quagga 

 and the zebras. 



Family Rhinocerotida;. There is now but one genus Rhinoceros, 

 species of which occur in Africa and in some parts of India and 

 Indo-Malaya. They are large heavy Ungulates, shy and noc- 

 turnal, fond of wallowing in water or mud, feeding on herbage, 

 shoots, and leaves. The skin is very thick, with scanty hair. 

 One or two median horns grow like huge warts from the snout 

 and forehead. The dentition is very variable, but the back 



teeth -^- are almost uniform, there are no upper canines, but 



4> 3 

 sometimes a large lower pair, there are a few incisors, but these 

 are often small and deciduous. 

 There are several entirely extinct families of Perissodactyla, such as 

 Lophiodontidae (Eocene), e.g., Lophiodon, Hyracotherium, 

 Systemodon,-ra. family perhaps ancestral to most of the 

 modern Perissodactyla. 

 PalEeotheriidse (Eocene to Miocene), e.g., Palmotherium and 

 Aruhitheriuvi. 

 Other remarkable types — 



Lambdotherium, Chalicotherium, Titanotherium, of elephantine 

 size, and the specialised Macraiichenia — are referred to 

 distinct families. 



Sub-Order Hyracoidea. 



Small Rodent-like Ungulates, represented by two genera, 

 — Hyrax, living in rocky regions in E. Africa and Syria, 



