52 PACHYDERMATA. 



so completely, or partially suppressed, that the triple division is 

 rendered very obscure, and it has commonly been found necessary, 

 by systematic authors, to include the whole series, in framing the 

 expression of the dental formula. In fact, till the appearance of 

 Professor Owen's Odontography, the normal division and theo- 

 retical signification of the different molar teeth in these genera 

 were not understood. 1 



In the ordinary Pachydermata, which the Rhinoceros may be 

 supposed to represent, there are normally four milk or deciduous 

 molars in both jaws, the hindmost of which has the complicated 

 form which characterizes the last true molar of the adult animal. 

 They are frequently reduced to three in other genera, by the sup- 

 pression of the anterior tooth, which is the most variable and rudi- 

 mentary in form. These teeth are succeeded vertically by an 

 equal number of premolars, the last of which is always of a sim- 

 pler form than the tooth of which it takes the place ; and they, in 

 like manner, are subject to a numerical reduction by the non- 

 development of one or more of the anterior teeth. Behind the 

 premolars are the true molars, the normal and developed number of 

 which is invariably three, this set being exempt from the partial 

 suppression to which the others are subject. They are dis- 

 tinguished from the premolars by greater complexity of form ; they 

 come into place like the milk molars, in antero-posterior succession, 

 and the first of the series is protruded and in use before the 

 appearance of the last premolar, which immediately precedes it 

 in position in the jaw. In the adult animal, in most genera,, the 

 whole of the premolars and true molars are simultaneously present 

 and in use in the jaws. A remarkable exception from this rule 

 takes place in the subgenus of the hog tribe, called Phacochcerus, 

 in which, in consequence of the complicated form and large size of 

 the last true molars, there is not room in the jaws to accommodate 

 the whole number at one time, and the first true molar is worn 

 down and pressed out before the last molar is protruded. This 

 last molar is gradually pushed forward, causing the anterior teeth to 

 be shed, so that the number of molar teeth in the upper jaw, which 



1 Ante, p. 11. 



