THE TEETH OF PERISSODACTYLE UNGULATA. 317 



In a brief survey, like that to which the present work is 

 necessarily confined, it will suffice to mention that there is 

 no great peculiarity about the incisors, or the canines, save 

 that the lower canine ranges with the lower incisors ; behind 

 the canine comes an interval, after which come the pre- 

 molars and molars, which are interesting, as being of simpler 

 pattern than those of most Ungulates, and it will be necessary 

 to very briefly allude to the various patterns characteristic 

 of ungulate teeth, with a view of showing how they may 

 have been derived the one from the other. 



In the Tapir four cusps are traceable, but ridges uniting 

 the two anterior and the two posterior cusps are strongly 

 developed, at the cost of the antero-posterior depression, i. e. 

 of one of the arms of the cross which separates the four cusps 

 in other quadricuspid molars. There is therefore left only a 

 deep transverse fissure (hence it is called a bilophodont 

 tooth), and the quadricuspid form is disguised. A low wall 

 on the outside of the tooth connects the two ridges. 



In the Hog we have a simple four-cusped molar, with a 



Fro. 134 J . 



crucial depression separating the cusps ; in the Hippopot- 

 amus the same pattern is repeated, though not quite so 

 simply, as each cusp is fluted in a definite manner. 



In Rhinoceros the two external cusps are united by a 



( J ) Grinding surfaces of upper molar series of a Rhinoceros, a. Posterior 

 sinus, which at a' has become an island, c. Posterior ridge, d. Anterior 

 ridge. 



