378 ANATOMY OF VERTEBRATES. 



preaching to a common Ungulate type. In the Proboscidian 

 group, the oldest species indicate retentions of type unknown 

 in the dentition of existing Elephants. A premolar, fig. 295, 

 p 3, displaces vertically the second deciduous grinder, ds, in some 

 Mastodons : and, that the third molar in the order of appearance, 

 ^/4, is also the last of the deciduous series, is indicated by the 

 contrasted superiority of size of the tooth, m i, that follows. The 

 great extent and activity of the processes of dental development 

 required for the preparation of the large and complex true molar 

 teeth, w T ould seem to exhaust the power in Proboscidians, which, 

 in ordinary Pachyderms, is expended in developing the vertical 

 successors of the deciduous teeth. In the miocene Mastodon 

 above cited, this normal exercise of the reproductive force was 

 not, however, wholly exhausted ; and one premolar, fig. 295, p 3, 

 of more simple form than its deciduous predecessor, was de- 

 veloped on each side of both jaws. Another mark of adhesion to 

 the archetype was shown by the development of two incisors in 

 the lower jaw in the young of some Mastodons, by the retention 



and development of one of these in- 

 ferior tusks in the male of the Mas- 

 todongiganteus of North America, and 

 by the retention of both in the Eu- 

 ropean Mastodon longirostris, Kaup. 

 No trace of these inferior homotypes 

 of the premaxillary tusks have been 



Deciduous teeth, Mastodon. i i p n t 



detected in the toetus or young 01 the 



existing elephants. In the gigantic Dinotlierium, the upper in- 

 cisors were suppressed, and the lower incisors were developed into 

 huge tusks, which curved down from the symphysis of the massive 

 lower jaw. 



The chief modifications of the marsupial dentition have already 

 been described and illustrated. The observed phenomena of the 

 development and change of the teeth led to the generalisation 

 that the marsupial differed from the placental Diphyodont 

 mammals in having four true molars, i. e., m :-* instead of 

 m -J:-J ; and also that they differed in having only three pre- 

 molars, i. e. p |:| instead of p -|:-J; the typical number of the 

 grinding series, ^;i, being the same ; and it was convenient for 

 comparison to symbolise them accordingly, in figs. 221-230. 

 Since, however, there is reason to conclude that m i in the pla- 

 cental Diphyodonts, as, e. g., figs. 259 and 294, is a continuation 

 of the deciduous series of molars, which might be symbolised as 



