694 



EVOLUTION OF THE HORSE. 



proves that there were two or more varieties or species^ to 

 which various names have been appHed. In 1884, M. 

 Ph. Thomas (Mc^moires de la Socide G^ologique de France) 

 having obtained, from a Pleistocene deposit in Algeria, 

 the lower jaw of a young animal of this order, which he 

 called Equus asinus atlanticus, noticed that its 3rd* 

 milk premolar had a small column or pillar of enamel, 

 at its posterior and external angle, where it is surrounded 

 with cement, and where it appears on the crown of the 

 tooth in the form of a small circle (Fig. 644). As the 

 position of this pillar is entirely different from that of 

 the Hipparion (p. 683), its presence shows that the 

 Hipparion is not an ancestor of this varietv of Equus 



Fig. 644. — 3rd lower left 

 premolar of Equus asinus 

 atlanticus (After Mar- 

 celliii Boule). 



Fig. 645. — 3rd lower left pre- 

 molar of Burchell's zebra 

 (Afta Marcelliii Boule). 



stenonis. While carefully investigating this subject, M. 

 Marcellin Boule (Bulletin de la Socid^ Ge^ologique de 

 France, 3e serie, tome xxvii., page 531, annee 1899), failed 

 to find this pillar in milk premolars of horses and asses 

 (tame or wild), and consequently inferred that E. asinus 

 atlanticus was not in their line of descent. On further 

 research, he discovered that the 3rd lower milk premolar 

 of Burchell's zebr,a (Fig. 645) has a pillar of enamel similar 

 in shape and position to that of E. asinus atlanticus ; and 

 that the teeth of this fossil resemble those of Burchell's zebra 

 much more closely than they do those of the horse and 

 ass. Also, he cites the important fact that the Algerian 

 deposits in which E. asinus atlanticus was found, contained 

 the remains of many tropical animals that, like the hip- 

 popotamus and rhinoceros, have long since quitted North 



* This 3rd premolar is the one which I have called the 4th ])remolar on page 49. 



