86 FAUNA ANTIQUA SIVALENSIS. 



include tlie whole series, in framing the expression of the 

 dental formula. In fact, till the appearance of Professor 

 Owen's 'Odontography,' the normal division and theoretical 

 signification of the difierent molar teeth in these genera were 

 not understood.* 



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 thi-ee in 

 other genera, by the suppression of the anterior tooth, which 

 is the most variable and i-udimentary in form. These teeth 

 are succeeded vertically by an equal number of premolars, 

 the last of which is always of a simpler 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 anteiior teeth. Behind the premolars are 

 the true molars, the normal and developed nimiber 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 iu 

 the subgenus of the hog tribe, called Phacochcerus, in which, 

 in consequence of the compUcated form and large size of 

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

 date the whole number at one time, and the first true molar 

 is worn down and pressed out before the last molar is pro- 

 truded. This last molar is gradually pushed forward, caus- 

 ing the anterior teeth to be shed, so that the number of 

 molar teeth in the upper jaw, which at one time m the 

 adolescent animal amounts to five on each side, is finally 

 reduced to one or two in the advanced age.^ Precisely ana- 

 logous conditions take place in the true elephants, in which 

 this kind of exception from the ordinary mode of dental suc- 

 cession is carried to the greatest known excess. 



Dinoilierium. — The first and most simple deviation from 

 the usual Pachydermatous type, in the dentition of the Pro- 

 boscidea, is presented by Binotlierium. In this genus only 

 two milk molars, viz. the penultimate and last, have been 



' Ante, p. 51. * Owen, Odontography, p. 550. 



i 



