Teeth are well developed in the vast majority of mammalia, and are of the greatest morphological and systematic importance, many extinct forms being known only by their teeth. Mammalian teeth differ from those of lower animals in various well-marked respects. (1) They are attached only to the maxillae, premaxillae and mandible, never to the palatines, pterygoids or other bones. (2) They frequently have more than one root. (3) They are always, except in some Odontoceti, placed in distinct sockets. (4) They are hardly ever ankylosed to the bone. (5) They are in most cases markedly heterodont. (6) They are commonly developed in two sets, the milk dentition and permanent dentition. It sometimes happens that teeth after being formed are reabsorbed without ever cutting the gum. This is the case, for instance, with the upper incisors of Ruminants. The form of mammalian teeth varies much, some are simple conical structures comparable to those of most reptiles, and these may either have persistent pulps, as in the case of the upper canines of the Walrus and the tusks of Elephants, or may be rooted as in most canine teeth. Some teeth have chisel-shaped edges, and this may be their original form, as in the human incisors, or may, as in those of Rodents, be brought about by the more rapid wearing away of the posterior edge, the anterior edge being hardened by a layer of enamel. Then, again, the crown may, as in the majority of grinding teeth, be more or less flattened. The various terms used in describing some of the forms of the surface of grinding teeth are defined on page 345. [Illustration FIG. 81. SKULL OF A YOUNG INDIAN RHINOCEROS (_R. unicornis_), SHOWING THE CHANGE OF THE DENTITION × 1/7. (Brit. Mus.) 1. nasal. 2. frontal. 3. parietal. 4. zygomatic process of squamosal. 5. jugal. _mI{1}._ milk incisor. _mc._ milk canine. _mpm{1}._ milk premolar. _I{1}._ first incisor. _c._ canine. _pm{2}_, _pm{3}_, _pm{4}_. 2nd, 3rd and 4th premolars. _m{1}_, _m{2}_. 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.