THE TEETH OF VNGULATA. 315 



Ungulata, or hoofed animals are grouped thus : 



(i.) Artiodactyles, or ) Hippopotamus. Pigs, Anoplotherium,&c., 

 even-toed Ungulata j Cows, Sheep, Deer, and other Ruminants. 



(ii.) Perissodactyles or) H oi-ses, Tapirs, Rhinoceros, PalaBotherium, 

 Ungulata with an / 

 odd number of toes ) 



The distinction between the two groups is strongly marked, if 

 living animals alone be considered ; but, as Professor Huxley has 

 pointed out, increasing knowledge of fossil forms is tending to 

 break down the line of demarcation. 



The recent forms bear, in all probability, but a very small pro- 

 portion to the extinct Ungulata, of which our knowledge is as yet 

 but fragmentary ; though the discoveries of Professors Marsh and 

 Cope in the " mauvaises terres " of Wyoming have brought to light 

 a very large number of strange and interesting ungulates ; and this 

 fragmentary condition of our knowledge makes it as yet impossible 

 to give a connected account of the dentition of ungulates, seeing 

 that the forms known to us are only isolated and often widely 

 separated links in the chain. 



The Teeth, of Perissodactyle Ungulates. Perisso- 

 dactyle (odd-toed) Ungulates are far less numerous than the 

 even-toed section, and among recent animals only comprise 

 the Horse, the Rhinoceros, the Tapir, and their allies. Their 

 premolars, or at least the last three of them, are equally 

 complex in pattern with the true molars ; and canines, tusk- 

 like but not very large, are of frequent occurrence. The 

 lower molars of almost all perissodactyles have a character- 

 istic form, their grinding surfaces being made up of two 

 crescentic ridges. 



The ungulate animals are all possessed of molar teeth, 

 which are kept in an efficient state of roughness by the 

 enamel dipping deeply into the crowns ; by the cusps, in 

 fact, being of very great depth. It consequently happens 

 that after the immediate apex is worn away, the flattened 

 working face of the tooth is mapped out into definite 

 patterns, which, on account of the light thus thrown upon 

 fossil remains, often consisting of little else than the teeth, 

 have been studied with great care. The result has been to 



