DISTRIBUTION OF MAMMALS IN TIME. 565 



Of the section of the Ungulates comprising the living Horse, 

 Zebra, and Ass (Solidungula), the earliest fossil example is the 

 Hipparion of the Miocene Rocks. This genus differed from 

 the existing Equidcz in the presence of two small toes with 

 hoofs, one on each side of the single functional toe, which 

 alone remains in living horses. In the Pliocene period appear, 

 for the first time, remains of horses which, like the present form, 

 possessed only a single toe encased in a single hoof. It is 

 interesting to observe that one of the Pliocene horses (Equus 

 curvidens] occurs in South America; though this continent cer- 

 tainly possessed no native horse at the time of its discovery by 

 the Spaniards. 



Of the Rhinoceridcz, a hornless species (Acerotheriuni) occurs in 

 Miocene and Pliocene strata ; but the best-known fossil species 

 is the two-horned woolly Rhinoceros (R. tichorhinus). This 

 curious species occurs in Post-pliocene deposits, and must have 

 ranged over the greater part of Europe. It was adapted to a 

 temperate climate, and, like the Mammoth, possessed a thick 

 covering of mixed wool and hair. This has been demonstrated 

 by the discovery of a frozen carcass in Siberia. 



Of the Hippopotamida the earliest-known species is the 

 Hippopotamus major of the Pliocene period. This form agreed 

 in all essential respects with the living H. amphibius of Africa, 

 but it must have ranged over the whole of southern Europe. 



Of the Suida, or Pig tribe, various extinct forms are known 

 from the Eocene and Miocene Rocks, where the family is 

 represented by the genera Cheer opotamus, Anthracotheriwn, 

 Hyopotamus, and Hippohyus. 



As regards the past existence of the Ruminants, the Cervidce, 

 or Stag tribe, is represented, for the first time in the Miocene 

 period, by the genus Dorcatherium. The best-known species, 

 however, of this family is the Megaceros Hibernicus, or so-called 

 Irish Elk (fig. 2 18), which is not a true Elk, but is intermediate 

 between the Fallow-deer and Reindeer. Of the Giraffe family 

 represented at the present day by a single African species 

 a form has been discovered in the Pliocene Rocks of Greece, 

 and has been described under the name of Helladotherium. 

 Somewhat similar forms have been found in the Pliocene de- 

 posits of the Sivalik Hills of India. 



The earliest-known Antelopes are Miocene, but the largest 

 and most extraordinary fossil examples of this family are two 

 gigantic four-horned Antelopes, which occur in the Pliocene 

 strata of the Sivalik Hills of India, and have been described 

 under the names of Sivatherium and Bramatherium. 



The Bovidce, or Ox tribe, has hitherto only occurred in rocks 



