338 MAMMALIA. 



Hipparion or Hippotherium (fig. 182). This genus occurs in the 

 Lower Pliocene of Europe, the Pliocene of North Africa, Persia, India, and 

 China, and in the Upper Miocene (Loup Fork Series) of North America. 

 It resembles Protohippus in the structure of the feet, but the teeth 

 are a little more complicated and hypsodont. The dental formula is 



i. 3, c. 1, pin. 4, m. 3 , 



, but the anterior upper premolar has no predecessor 

 i. 3, c. 1, pm. 3, m. 3 ' 



and is shed before the animal is adult. The diastemata in the dental 

 series are as in the modern Equus, but the skull differs from that of exist- 

 ing horses in the presence of a large deep fossa in the maxillary region 

 on each side. The best known species are Hipparion gracile and closely 

 similar forms from the Lower Pliocene of Eppelsheim, Hesse Darmstadt, 

 of Pikermi, Greece, and of many other European localities. They are 

 smaller than the ordinary Equus caballus, scarcely one-and-a-half metres 

 high at the shoulder. 



Hippidium. A South American horse with comparatively simple 

 molar teeth, and a very long slit in each side of the facial region of the 

 skull beneath the elongated and attenuated nasal bones. From superficial 

 deposits in Brazil and Bolivia, and from the Pampa Formation of 

 Argentina. 



Equus (fig. 182). The true monodactyl horse, with mere splint bones 

 to represent lateral digits, appears first, so far as known, in the Lower 

 Pliocene of the Siwalik Hills, India (Equus sivalensis). In the Upper 

 Pliocene of Europe and northern Africa, one species (E. stenonis) is very 

 widely distributed ; and this occurs in the Cromer Forest Bed of Norfolk. 

 These early species exhibit the depression on each side of the facial region, 

 which is so characteristic of Hipparion, but absent in their successors. 

 In the Pleistocene there is evidence of horses throughout Europe, northern 

 Africa,. Asia, and North and South America; and a large proportion of 

 the remains can scarcely be distinguished from the corresponding parts 

 of the existing E. caballus. It is strange that the horse should once 

 have ranged over the whole of the New World, and actually have been 

 proved a contemporary of primitive man in North America, and } r et 

 have become quite extinct on that continent before its colonization from 

 Europe. 



Sub-Order 10. Artiodactyla. 



The existing even-toed hoofed animals, or ARTIODACTYLA, 

 are subdivided into four very distinct groups, namely, the 

 Suina (pigs, peccaries, and hippopotamus), Tylopoda (camels 

 and llamas), Tragulina (chevrotains), and Pecora (deer, sheep, 

 and cattle). As in the case of the Perissodactyla, however, 

 so large a number of the extinct predecessors of the 



