Fig. 81. Skull of a young Indian Rhinoceros (R. unicornis),

SHOWING THE CHANGE OF THE DENTITION × 1/7. (Brit. Mus.)

1. nasal.mc. milk canine.
2. frontal.mpm_{1}. milk premolar.
3. parietal.I_{1}. first incisor.
4. zygomatic process ofc. canine.
squamosal.pm_{2}, pm_{3}, pm_{4}. 2nd, 3rd and
5. jugal.4th premolars.
mI_{1}. milk incisor.m1, m2. first and second molars.

The teeth of the Aard Varks are compound, and differ completely from those of all other mammals (see p. 425).

As a rule, the higher the general organisation of an animal the better are its milk teeth developed, and the more do they form a reproduction on a small scale of the permanent set. This fact is well seen in the Primates, Carnivora and Ungulata. The method of notation by which the dentition of any mammal can be briefly expressed as a formula has been already described. The regular mammalian arrangement of teeth for each side is expressed by the formula

i 3/3 c 1/1 pm 4/4 m 3/3 × 2; total, 44.

Monotremata. In Echidna teeth are quite absent. In the young Ornithorhynchus[146] functional molar teeth of a multi-tubercular type resembling those of some Mesozoic mammalia are present, but in the adult they disappear, their office being discharged by horny plates.

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.