432 UNGULATA 
very large, as in Elephas, sometimes with longitudinal bands of 
enamel, more or less spirally disposed. Lower incisors variable ; 
when present comparatively small and straight, sometimes per- 
sistent, sometimes early deciduous, and in some species never 
present. Grinding surface of molars with transverse ridges, the 
summits of which are divided more or less into conical or mam- 
millary cusps, and often with secondary or additional cusps between 
and clustering against the principal ridges; enamel thick; cement 
very scanty, never filling up the interspaces between the ridges. 
The third, fourth, and fifth cheek-teeth (ie. the last milk-molar, 
and the first and second molars) having the same number of ridges,! 
which never exceeds five. 
In the upper jaw the incisors, though of large size, were 
apparently never so much curved as in some species of Elephant, 
and they often have longitudinal bands of enamel, more or less 
spirally disposed upon their surface, which are not met with in 
Elephants. Lower incisors were present throughout life in some 
species, which have the symphysis of the lower jaw greatly elon- 
gated to support them (as in If. angustidens, M. pentelici, and M. 
longirostris). Inthe common North American species (Af. americanus) © 
the mandibular symphysis is short, but it may have a small incisor 
on one side. In other species no inferior tusks have been found, 
at all events in adult life (see figure of MZ. arvernensis). 
The molar teeth increase in size from before backwards, but as 
many as three of these teeth may be in place in each jaw at one 
time. There is in many species a true vertical succession, affecting 
either the third, or the third and second, or (in M. productus) the 
first, second, and third of the six molariform teeth. These three 
are therefore reckoned as milk-molars, and their successors as pre- 
molars, while the last three, which are never changed, correspond 
to the true molars of those animals in which the typical dentition 
is fully developed. The study of the mode of succession of the 
teeth in the different species of Mastodons is particularly interest- 
ing, as it exhibits so many stages of the process by which the very 
anomalous dentition of the modern Elephants may have been 
derived by gradual modification from the typical heterodont and 
diphyodont dentition of the ordinary mammal. It also shows that 
the anterior molars of Elephants do not correspond to the pre- 
molars of other Ungulates, but to the milk-molars, the early loss of 
which in consequence of the peculiar process of horizontal forward- 
1 This, and the larger number of ridges in the latter, are the only absolute 
distinctions which Falconer could find between Mastodon and Elephas (Paleont. 
Memoirs, ii. p. 9), and it is clear that they are somewhat arbitrary. The line 
between the two genera is drawn at this point more as a matter of convenience 
for descriptive purposes than as indicating any great natural break in the 
sequence of modifications of the same type. 
