In Myrmecobius five or six molar teeth occur on each side, and the total number of teeth reaches fifty-two or fifty-six. The teeth bear rows of tubercles, and resemble those of the Multituberculate mesozoic Mammals[150], more than do those of any other living form. Calcified teeth have recently been described[151] in Myrmecobius earlier than the functional or milk set. This would relegate the milk teeth of mammals in general to a second series, and the permanent teeth to a third. In Notoryctes the dental formula[152] is given as i 3/2 c 1/1 pm 2/3 m 4/4, total 40. The canines are small, and the anterior molars have strongly developed cusps, and much resemble those of Chrysochloris (Insectivora).
Among the diprotodont types the Phascolomyidae, or Wombats, have a dentition recalling that of the Rodents. All the teeth grow from persistent pulps, and the incisors have enamel only on the anterior surface as in Rodents. The dental formula is i 1/1 c 0/0 pm 1/1 m 4/4, total 24. There are indications of a vestigial second pair of incisors.
The Macropodidae, or Kangaroos, have a herbivorous dentition with the formula i 3/1 c (0—1)/0 pm 2/2 m 4/4. The incisors are sharp and cutting, and are separated by a long diastema or gap from the molars, which have their crowns marked by ridges or cusps. There are indications of several vestigial incisors.
Coenolestes, a remarkable form recently described from America, belongs to the diprotodont section, and is the only living member of the section known outside the Australian region[153]. An exceptional dentition is seen in the case of the extinct Thylacoleo, in which the functional teeth are reduced to two pairs; one pair of large cutting incisors and one of compressed sharp-edged premolars.
Edentata. Some Edentata, viz. the ant-eaters (Myrmecophagidae) are, as far as is known, absolutely toothless at all stages of their existence; being the only mammals except Echidna in which no tooth germs have been discovered; others, viz. the Manidae, though showing foetal tooth germs, are quite toothless in post-foetal life; others, viz. some of the armadillos, have the largest number of teeth met with in land mammals. The teeth are homodont except in the Aard Varks, and grow from persistent pulps. In the sloths (Bradypodidae) and the Megatheriidae, there are five pairs of teeth in the upper and four in the lower jaw. The teeth of sloths consist of a central axis of vasodentine, surrounded firstly by a thin coating of hard dentine, and secondly by a thick coating of cement.
In no living Edentate have the teeth any enamel; it has, however, been described as occurring in certain early Megatheroid forms from S. America[154], and an enamel organ has also been discovered in an embryo Dasypus[155]. In the Armadillos (Dasypodidae) the number of teeth varies from 8/8 or 7/7 in Tatusia, to upwards of 25/25 in Priodon, which therefore may have upwards of a hundred teeth, the largest number met with in any land mammal. In Tatusia all the teeth except the last are preceded by two-rooted milk teeth. The Aard Varks are diphyodont, and milk teeth are also known in a species of Dasypus, but with these exceptions Edentates are, as far as is known, monophyodont. In Glyptodon the teeth are almost divided into three lobes by two deep grooves on each side.
The Aard Varks (Orycteropodidae) are quite exceptional as regards their teeth, which are cylindrical in shape, and are made up of a number of elongated denticles fused together. Each denticle contains a pulp cavity from which a number of minute tubes radiate outwards. These teeth are diphyodont and somewhat heterodont, eight to ten pairs occur in the upper jaw and eight in the lower, but they are not all in place at one time. The last three teeth in each jaw are not preceded by milk teeth[156].
Sirenia. The teeth of Sirenia show several very distinct types, the least modified being that of the extinct Halitheriidae, which have large incisors in the upper jaw, and five or six pairs of tuberculated grinding teeth in each jaw, the anterior ones being preceded by milk teeth.
In both the living genera the dentition is monophyodont. In Manatus the dentition is i 2/2 pm and m 11/11. The incisors are vestigial, and disappear before maturity. The grinding teeth have square enamelled crowns marked by transverse tuberculated ridges. They are not all present in the jaw at the same time. In Halicore the upper jaw bears a pair of straight tusklike incisors; in the male these have persistent pulps and project out of the mouth; in the female they soon cease to grow and are never cut. They are separated by a long diastema from the grinding teeth which have tuberculated crowns and are 5/5 or 6/6 in number, but are not all in place at once. Several other pairs of slender teeth occur in the young animal, but are absorbed or fall out before maturity. In Rhytina teeth are altogether absent.
Cetacea.