Amblypoda. Two of the best known forms belonging to this extinct group differ much as regards dentition. For while Coryphodon has the regular dental formula, and the canines of both jaws of moderate size, in Uintatherium the dentition is very specialised, there are no upper incisors, and the upper canines form a pair of enormous tusks. The grinding teeth form a continuous series marked by V-shaped ridges and the dental formula is i 0/3 c 1/1 pm 3/3 m 3/3 total 34.
Proboscidea. The incisors are composed entirely of dentine and have the form of conical tusks projecting greatly from the mouth. In living forms they are confined to the upper jaw, in some species of the extinct Mastodon however they occur in the lower jaw also. In Dinotherium they are probably absent from the upper jaw, but form a pair of downwardly and backwardly-directed tusks growing from the elongated symphysis of the mandible.
The grinding teeth in the various Proboscidea show a very remarkable series of modifications. In Dinotherium they are bilophodont or else are marked by three straight transverse ridges. The dental formula is i 0?/1 c 0/0 pm 2/2 m 3/3, and the teeth have the normal method of succession. In Mastodon as in Dinotherium the grinding teeth are marked by transverse ridges, but the ridges are subdivided into conical or mammillary cusps, and similar cusps often occur between the ridges. These cusps are covered with very thick enamel and the spaces between them are not filled up with cement. There are six of these grinding teeth for each side of each jaw but only three are in place at once. The first three are milk teeth as they may be succeeded vertically by others.
In the true Elephants the number and depth of the enamel folds is much increased, and the spaces between the folds are filled up with cement. A very complete series of extinct forms is known with teeth intermediate in character between those of Mastodon and those of the Mammoth and living elephants. The dental formula of Elephas is
di 1/0 i 1/0 c 0/0 dm 3—4/3—4 m 3/3.
Sir W.H. Flower describes[162] the mode of succession of teeth in Elephants as follows: "As regards the mode of succession that of modern Elephants is as before mentioned very peculiar. During the complete lifetime of the animal there are but six molar teeth on each side of each jaw with occasionally a rudimentary one in front, completing the typical number of seven. The last three represent the true molars of ordinary mammals, those in front appear to be milk molars which are never replaced by permanent successors, but the whole series gradually moves forwards in the jaw, and the teeth become worn away and their remnants cast out in front while development of others proceeds behind. The individual teeth are so large and the processes of growth and destruction by wear take place so slowly, that not more than one or portions of two teeth are ever in place and in use on each side of each jaw at one time, and the whole series of changes coincides with the usual duration of the animal's life. On the other hand the Dinotherium, the opposite extreme of the Proboscidean series, has the whole of the molar teeth in place and use at one time, and the milk molars are vertically displaced by premolars in the ordinary fashion. Among Mastodons transitional forms occur in the mode of succession as well as in structure, many species showing a vertical displacement of one or more of the milk molars, and the same has been observed in one extinct species of Elephant (E. planifrons) as regards the posterior of these teeth."
In the Tillodontia the grinding teeth are of Ungulate type, while the second incisors are large and grow from persistent pulps, so as to resemble those of Rodents.
Rodentia have a most characteristic and very constant dentition, the common dental formula being
i 1/1 c 0/0 pm (0—1)/(0—1) m 3/3, total 18 or 20.
The incisors always have chisel-like edges and persistent pulps, and are separated by a wide diastema from the premolars. Canines are always absent, and there are generally three grinding teeth not preceded by milk teeth; their surface may be grooved, or may be bunodont. Teeth are most numerous in the Duplicidentata (Hares and Rabbits), in which the formula is i 2/1 c 0/0 pm 3/2 m 3/3, total 28, and fewest in Hydromys and certain other forms, in which the formula is i 1/1 c 0/0 pm 0/0 m 2/2, total 12. The hares and rabbits are the only rodents which have well developed deciduous incisors, though a vestigial milk incisor has been described in the Mouse (Mus musculus). The last upper molar of Hydrochaerus is very complicated, its structure approaching that of the teeth of Elephants.