ELEPHANTIDA 433 
moving succession does not require, or allow time for, their replace- 
ment by premolars. It must be noted, however, that, in the 
Mastodon in some respects the least specialised in tooth-structure, 
the A. americanus of North America, no vertical succession of the 
A 
Sere 
Fic. 186.—Restoration of the skeleton of Mastodon arvernensis, from the Pliocene of Europe. 
(After Sismonda.) 
molars has yet been observed, although vast numbers of specimens 
have been examined. 
The Mastodons have fewer ridges on their molar teeth than 
the Elephants; the ridges are also less elevated, wider apart, have 
a thicker enamel-cover- 
ing, and scarcely any 
cement filling up the 
space between them. 
Sometimes (as in Jf. 
americanus) the ridges 
are simple transverse 
wedge-shaped — eleva- 
tions, with straight or 
concave edges. In 
other species the sum- Fic. 187.—Oblique side and crown view of the last upper 
mits of the ridges are molar of Mastodon arvernensis. (From Owen.) 
more or less subdivided into conical cusps, and may have accessory 
cusps clustering around them (as in Jf. arvernensis, see Fig. 187). 
When the apices of these are worn by mastication, their surfaces 
present circles of dentine, surrounded by a border of enamel, and 
as the attrition proceeds different patterns are produced by the 
union of the bases of the cusps, a trilobed or trefoil form being 
characteristic of some species (Fig. 188). 
28 
