174 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



PERISSOBACTYLA. 



The probably tritubercular origin of tbe molar teetb of the odd-toed 

 Ungulates or Perissodactyla has been discussed in the case of horses, 

 rhinoceroses, and tapirs on pp. 72, 75, 85-87. The key to the 



parastyle - 

 paracone - 



protoconule 

 proiocone 



protocontd hypoconid 



metaconid entoconid 



FIG. 159. Upper and lower molar of Hyrocotlteriuin leporinum from the London Clay (Spar- 

 nacien) Lower Eocene, showing the fundamental ancestral pattern of the Perissodactyl molars, 

 i.e. Upper molars with four well rounded cusps and two small conules, the rudiments of the proto- 

 and metaloph, lower molars with four main cusps, lacking the paraconid. and with an incipient 

 hypoconulid. The hypocone and metaconule are apparently twin cusps, the hypocone not being 

 derived from the cingulum. xf. After Owen. 



rnesostyle 



in, Irifstijle 



protolojih--\- 



I'lttlixtllltll- 



KII; 100. The Secondary cusps of an Ungulate Molar. A. Anchithc, :>',. upper. B. Mery- 

 is, lower. The Primary cusps are indicated by abbreviations. 



origin of the primitive sexitubercular superior molars, from which the 

 elaborate pattern of all these teeth were evolved, is to be found not 

 only among the Condylarthra (pp. 168-170), but also in the study of 

 the various types of teeth in the horses themselves. From this sexi- 

 tubercular type the crown evolves by a bunoid, lophoid, or selenoid 



