856 PHYLUM CHORDATA : CLASS MAMMALIA 



progress shows an increase of size, a diminution in the number of 

 digits, an increased folding of the back teeth, and other differentiations. 

 The lilocene Phenacodus is regarded by some as near the origin of the 

 stock, it had five complete digits on each foot ; Hyracotherium and 

 Sysiemodon 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 ; in the Pliocene also, the modern forms 

 appeared. 



The living species are the horses {Equus caballus), apparently 

 originating 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. przevalskii) ; the donkey {E. asinus) of 

 African origin ; the wild asses of Africa and Asia ; the striped African 

 species— the zebras and the (exterminated) quagga. 



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 as 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 — 



Lophiodontidaj (Eocene), e.g. Lophiodon, Hyracotherium, Sys- 



temodon — a family perhaps ancestral to most of the modern 



Perissodactyla. 

 Palaeotheriidae (Eocene to Miocene), e.g. Palceotherhim and 



Anchitherium. 

 Other remarkable types — Lambdotherium, Chalicotherium, Titano- 

 therium, of elephantine size, and the specialised Macrauchcnia 

 — are referred to distinct families. 



Sub-Order Hyracoidea 



An isolated order of small Rodent-like Ungulates, repre- 

 sented by Hyrax {Procavia) and Dendrohyrax, living in 

 rocky regions and on trees in Africa and Syria. The 

 species (14) are adept climbers. 



The upper incisors have persistent pulps, and are curved 

 as in Rodents, but they are sharply pointed, not chisel- 

 edged. The outer lower incisors are straight, and have 

 trilobed crowns, There are no canines in the second set, 



