398 



PALAEONTOLOGY. 



derivative hypothesis of species, might be the transitional 

 form between the upper eocene Palreotheres (PaloplotheHum 

 and Ancliitherium) and the modern horse. 



Species of true Equus with the spurious hoofs suppressed, 

 and the interlobal column blended with the body 

 of the tooth (fig. 151, m i and 2), first make their 

 appearance in pliocene beds. Equus plicidens, 

 so called because enamel-ridges on the teeth are 

 more plaited than in the recent horse, occurs in 

 pliocene brick-earth, and in the Oreston Lime- 

 stone caves, England ; a similar species has also 

 been found associated with Mastodon and Ceto- 

 tolites in a pliocene deposit at Newberne, North 

 Carolina. In South American formations of 

 similar age, a horse (Equus curvidens) with 

 grinding teeth more bent than usual, has left 

 its remains along with those of the Mega- 

 therium. Both kinds of aboriginal American 



m 



Fig. 150. 



Bones of foot. 

 Hipparion. 



horse became extinct with the larger quadru- 

 peds with which they were associated in the two divisions of 

 that great continent. The Equus fossilis of the crag and drift 

 gravel of England appears to have had grinders with a less 



Fig 151. 



Dentition of upper jaw, horse. 



transverse diameter than our modern variety of similar size. 

 Fossil equine teeth, of the size of those in the zebra and ass, 

 have also been found in pliocene and later deposits of both 



