In elephants there are only two, the last milk-molar and the first true molar (or the third and fourth of the whole series), which are alike in the number of ridges; whereas in mastodons there are three such teeth, the last milk-molar and the first and second molars (or the third, fourth and fifth of the whole series).
The dentition of the kangaroos, functionally considered, thus consists of sharp-edged incisors, most developed near the median line of the mouth, for the purpose of cropping herbage, and ridged or tuberculated molars for crushing.
In Xerus itself, which is represented by the terrestrial African spiny squirrels, the ears are short, there are only two teats, and flat spines are mingled with the fur; while the skull, and more especially the frontals, is elongated, with a very short post-orbital process, and the crowns of the molars are taller than usual (see Spiny Squirrel).
It also shows that the anterior molars of elephants do not correspond to the premolars of other ungulates, but to the milk-molars, the early loss of which in consequence of the peculiar process of horizontal forward-moving (From Owen.) FIG.
These are for the most part large antelopes, with long cylindrical horns, which are present in both sexes, hairy muzzles, no face-glands, long tufted tails and tall thick molars of the ox-type.
The well-known Indian palm-squirrel, Funambulus palmarum, typifies an Indo-Malay genus allied to Xerus in skull-characters but with molars more like those of Sciurus.
Outer columns of upper molars similar, the hinder ones not flattened; ridges of lower molars oblique or directly transverse, a third ridge to the last molar in the earlier forms. The Lophiodontidae, which date from the Eocene, come very close to Hyracotherium in the horse-line; and it is solely on the authority of American palaeontologists that the division of these early forms into equoids and tapiroids is attempted.
The cheek-teeth are short-crowned, generally with no cement, the upper molars having a W-shaped outer wall, from which proceed two oblique transverse crests, while the lower ones carry two crescents.
As stated above, the lower molars have the crown formed by a pair of crescents; the last having no third lobe.
The hollow tympanic bullae, they have the clavicles imperfect, the first front toe opposable to the rest, the temporal region of the skull roofed with bone, and the crowns of the molars with cusps arranged in rows but eventually covered by a layer of enamel.