434 eee cem: E Em 
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. Faleoner, and are 
Fic. 188.—Grinding surface of the partially worn last three in number, hame- 
left lower milk-molar of Mustofon angustidens, from the Jy the last milk-molar 
Upper Miocene of India. The lower side of the figure is ~ Sane eee ene 
the outer border of the tooth. and the first and se cond 
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 Tr ilophodon (of which an intermediate molar is shown 
in Fig. 188), and the latter that called Zetralophodon by Dr. Faleoner. 
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 hy the 
formule 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 Jfustedon 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 i. americanus, M. ohioticus, and MW. giganteus 
—survived to a late Pleistocene period. 
The range in space will be best indieated by the following list 
of some of the better known species. (1) Trilophodont series 
AL. angustidens,! borsoni, pentelici, turicensis, from Europe ; 1. falconert 
1 Also found beyond the extreme north-western froutier of India. 
