MARSUPIALIA[147] have a heterodont dentition, which has generally been regarded as almost monophyodont, the only tooth which has an obvious deciduous predecessor being the last premolar. The researches of Röse[148] and Kükenthal[149] tend to show that the teeth of Marsupials are developed in the same way as in other mammals, and are diphyodont. In the case of the premolars, teeth which are homologous with the permanent teeth of other mammals begin to develop as lateral outgrowths from the milk teeth, but afterwards become absorbed, so that the teeth which actually persist belong to the milk series. The last premolar, however, does as a rule develop and replace its milk predecessor; sometimes, however, as in _Didelphys_, it takes its place among the milk molars without replacing one of them. The types of dentition characteristic of the different groups of placental mammals may mostly be paralleled among the Marsupials. Thus among the polyprotodont forms the Didelphyidae or opossums, and some of the Dasyuridae, such as _Sarcophilus_ and _Thylacinus_, have a typical carnivorous dentition with small incisors, large canines, and molars with pointed compressed crowns. The dental formula of _Thylacinus_, is _i_ 4/3 _c_ 1/1 _pm_ 3/3 _m_ 4/4, total 46. 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.