ELEPHANT AND MASTODON. 53 



at one time in the adolescent animal amounts to five on each side, 

 is finally reduced to one or two in the advanced age. 1 Precisely 

 analogous conditions take place in the true Elephants, in which 

 this kind of exception from the ordinary mode of dental succession 

 is carried to the greatest known excess. 



Dinotherium. — The first and most simple deviation from the 

 usual Pachydermatous type, in the dentition of the Proboscidea, is 

 presented by Dinotherium. In this genus, only two milk molars, 

 viz., the penultimate and last, have been met with in both jaws, 

 one or two of the anterior teeth of this set being suppressed. 

 The last milk molar, above and below, is three-ridged, while the 

 penultimate has only two ridges. These teeth are replaced ver- 

 tically by an equal number of premolars, which represent the 

 penultimate and last, the two anterior teeth of this series being 

 also suppressed. The last premolar, as well as the penultimate, is 

 only two-ridged, conforming to the ordinary rule of being simpler 

 in form than the milk molar which it succeeds. Of the three true 

 molars, the first or antepenultimate, in both jaws, is three-ridged, 

 repeating the complex form of the last milk molar, while the 

 penultimate and last are only two-ridged. This is a very remark- 

 able anomaly, of which no other example is known among the 

 Pachydermata, as it is commonly the last true molar which 

 reiterates the form of the last milk molar. The first true molar is 

 protruded and in use before the last milk molar is shed, so that in 

 the adolescent animal there are two contiguous teeth, which have 

 each three ridges. We have, in this circumstance, the first 

 essential proboscidean character, which at once distinguishes 

 Dinotherium from the Tapirs and allied genera, and indicates its 

 near relations to Mastodon. 



In regard to the number of teeth which are in place and in use 

 at the same time, Dinotherium is less aberrant than even Phaco- 

 chasrus, as the two premolars and three true molars in the adult 

 animal are simultaneously present in both jaws. The molar formula 

 Dinotherium is therefore 2 premol. + 3 mol. = 5 in each side 

 of both jaws ; and the number of ridges in the different teeth, 



1 Owen, Odontography, p. 550. 



