1360 CLASS MAMMALIA. 



either one (fig. 1232) or three in number ; the ulna and fibula are 

 incomplete ; and the meso- and entocuneiform of the tarsus are 

 united. The plane of wear of the cheek-teeth becomes nearly 

 smooth, instead of being raised into ridges as in the preceding 

 families ; and the summits of the incisors have an infolding of the 

 enamel extending some distance into the crown. One of the most 

 generalised forms is Protohippus or Merychippus of the Lower 

 Pliocene of North America, in which the permanent molars re- 

 semble those of the generalised species of Equus, but have shorter 

 crowns, while the milk-molars approximate more nearly to the true 

 molars of Anchitherium. The next genus is Hipparion (Hippo- 



therium). The dentition is /. ^, C. -, Pm. -, M. - ; but the first 



3i 3 3 



upper cheek-tooth, which has no predecessor and appears with 

 the milk series, is shed before the animal is adult. The upper 

 cheek-teeth (fig. 1233) at first sight seem to differ very widely 



Fig. 1233. — Three right upper cheek-teeth of Hipparion; from the Pliocene of India, «, 

 Posterior, and b, Anterior outer crescent ; c, Anterior, and d, Anterior inner crescent ; e, Ante- 

 rior, and f, Posterior pillar. 



from those of the Palceotherium type (fig. 1229), but a closer 

 examination will show that the outer portions marked a and b cor- 

 respond to the outer wall of the more generalised tooth ; while the 

 portions c and e, and d and / respectively represent the first and 

 second transverse ridges of the same. These ridges have, however, 

 united together in a crescent-like form, and enclose between them 

 and the outer wall a pair of islands surrounded by a plicated wall of 

 enamel and filled with cement. The terms which it is convenient 

 to apply to the Equine molar are indicated in the accompanying 

 figure ; and the distinctive feature of the upper teeth of Hipparion 

 is that the anterior pillar (e) is disconnected from the anterior inner 

 crescent (c) for at least three-quarters of its height, so that it nor- 

 mally appears on the worn crown as an isolated oval (fig. 1233). 

 There are normally three digits to each foot, but in the Pliocene 

 Indian H. antilopinum they are apparently reduced to one ; this 



