I362 CLASS MAMMALIA. 



have a narrow anterior pillar in the upper jaw, and thus show their con- 

 nection with Hipparion. In E. sivalensis, of the Pliocene of India, and 

 E. quaggoides of that of Italy, this pillar becomes rather more widened ; 

 and in the Pleistocene E. namadicus of India, as well as in E. curvide?ts 

 of that of Brazil, and all the existing members of the genus (fig. 1234) 

 the grinding surface of this pillar becomes greatly widened in the 

 antero-posterior direction. It is noteworthy that E. sivalensis is found 

 in the same beds as those containing Hipparionj and that the existing 

 E. caballus is apparently the common species of the European Pleisto- 

 cene, although it is not improbable that the Asiatic E. onager may also 

 occur in the same deposits. In Southern India, where no living species 

 are found, the remains of the existing African E. asinus and of a larger 



Fig. 1235. — Skull of the Horse {Equzts caballus). Reduced. 



undetermined form occur in the Pleistocene cave-deposits of Madras. 

 Finally, it should be mentioned that some of the South American Tertiary 

 forms have been separated under the name of Haplohippus. 



Genealogy of the Horse. — Allusion has been incidentally made 

 in the preceding paragraphs to the genealogy of the genus Equus, but 

 since this is one of the best instances of evolution among the higher 

 Mammals yet worked out it is advisable that it should be noticed rather 

 more fully. The top of the series is the Equus caballus group, in which 

 the dentition is of the most specialised type, and which descends in India 

 to the topmost Pliocene ; then we have the E. Stenonis and E. principalis 

 group, in which the molars become more like those of Hipparion ; and 

 finally the so-called Hippidium. The structure of the foot is shown in 

 fig. 1236, D ; the great size of the phalangeals and the metapodial of the 

 functional digit being very noticeable. The earliest occurrence of the genus 

 is in the Pliocene of India. From the E. Stenonis group to Hipparion 

 is but a step, the transitional species being H. antilopinum of the Plio- 

 cene of the Siwalik Hills in which the lateral digits were apparently 

 wanting ; in the other species (fig. 1236, c) the lateral digits are of 

 small size, and the middle one is relatively more slender than in Equus. 

 Protohippus connects Equus by the structure of its milk-molars with 

 the Miocene Anchitherium : and in the latter the teeth have become 

 brachydont, the third lower molar has a small third lobe, and the lateral 



