434 



UNGULATA 



As already mentioned, certain of the molariform teeth of the 



middle of the series in 

 Mastodons have the 

 same number of princi- 

 pal ridges, those in 

 front of them having 

 fewer and those behind 

 a greater number. 

 These teeth were dis- 

 tinguished as " inter- 

 mediate " molars by 

 Dr. Falconer, and are 



FIG. 188. Grinding surface of the partially worn last three ill number, name- 

 left lower milk -molar of Mastodon angustidens, from the }y the last milk-molar 

 Upper Miocene of India. The lower side of the figure is j .-, r- 

 the outer border of the tooth. and the first and S6COnd 



true molars (or the 



third, fourth, and fifth of the whole series). The number of ridges 

 on these intermediate molars is nearly always three or four, and the 

 tooth in front has usually one fewer and that behind one more, so 

 that the ridge-formula of most Mastodons can be reduced either to 

 1, 2, 3, 3, 3, 4, or 2, 3, 4, 4, 4, 5. The former characterises the 

 section called Trilophodon (of which an intermediate molar is shown 

 in Fig. 188), and the latter that called Tetralophodon by Dr. Falconer. 

 These divisions are very useful, as under one or the other all the 

 present known species of Mastodon can be ranged, but observations 

 upon a larger number of individuals have shown that the number 

 of ridges upon the teeth is not quite so constant as implied by the 

 formulae given above. Their exact enumeration is even difficult in 

 many cases, as " talons " or small accessory ridges at the hinder end 

 of the teeth occur in various stages of development, until they take 

 on the character of true ridges. Transitional conditions have* also 

 been shown, at least in some of the teeth, between the trilophodont 

 and the tetralophodont forms, and again between the latter and 

 what has been called a " pentalophodont " type, which leads on 

 towards the condition of dental structure characteristic of the true 

 Elephants. 



The range of the genus Mastodon in time was from the middle 

 of the Miocene period to the end of the Pliocene in the Old World, 

 when it became extinct ; but in America several species especially 

 the one best known, owing to the abundance of its remains, which 

 has been variously called M. arnericamis, M. ohioticus, and M. giganteus 

 survived to a late Pleistocene period. 



The range in space will be best indicated by the following list 

 of some of the better known species. (1) Trilophodont series 

 M. angustidens, 1 borsoni, pentelici, turicensis, from Europe ; M. falconen 

 1 Also found beyond the extreme north-western frontier of India. 



