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 (i.e. the last milk-molar, 

 and the first and second molars) having the same number of ridges, 1 

 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 M. angustidens, M. pentelici, and M. 

 longirostris). In the common North American species (M. 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 M. arvemensis). 



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. produdus) 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 (Palccont. 

 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 

 secpiencc of modifications of the same t\\ e. 



