200 MAMMALIAN DENTITION 



Suffice it to say that so far as teeth are concerned the premolars 

 and molars in the Artiodactyls are never alike as a series and 

 only rarely does the last premolar ever become molariform. In 

 the Perissodactyls, on the contrary, all the premolars except 

 the first are molariform in size and in appearance. Further, 

 throughout the entire history of the two orders so far as it is 

 known at present these differences have existed. In the most 

 ancient examples of the Artiodactyls the premolars are simple, 

 the fourth being at most bicuspid; the upper molars are trituber- 

 cular and the lowers very like typical tuberculo-sectorial teeth, 

 whereas the oldest Perissodactyls already show a molariform 

 appearance in the posterior premolars, quadritubercular upper 

 molars and lowers with commencing lophs. 



ARTIODACTYLA, THE EVEN-TOED UNGULATES 



Included within the order Artiodactyla are such diverse ani- 

 mals as Pigs and Hippopotami ; Camels and Llamas ; Deer and 

 Giraffes; Antelopes, Sheep, Oxen and Goats; an aggregation 

 strange and varied enough yet in structure showing remark- 

 able unity and clearly defined distinctions from members of all 

 other orders. 



Among these creatures we find two types of molar, the buno- 

 dont and the selenodont, together with varied degrees of hyp- 

 sodonty. It is often said that of the two types the bunodont is 

 the more primitive and that from it the selenodont differenti- 

 ated, but however this may be, the latter type dates back to 

 the dawn of Artiodactyl history. In other orders we have 

 already seen the formation of the selenodont type through the 

 utilization of styles but it will be recalled that this develop- 

 ment was true of the upper molars only. The production of 

 the crescents of the lower molars was rather directly from the 

 drawing out of the edges of primitive cusps possessing the 

 shape of a three sided pyramid. It is probable that the Artio- 

 dactyl crescents were formed in a similar manner for the most 

 ancient members of this order and of the Perissodactyla also 



