teeth in which the grinding surface is raised into a series of two, to many, tubercles sharper or blunter as the case may be;—sharper and fewer at the same time in carnivorous and especially in insectivorous types, more abundant in omnivorous animals. To this form of tooth the term "bunodont" is applied. There is no doubt that this is the earliest type of tooth; but whether the fewer or the more cusped condition is the primitive one is a question that is reserved for consideration at the end of the present chapter. The other type of grinding tooth is known as "lophodont." This is exemplified by such types as the Perissodactyla and Ungulates generally, and by the Rodents. The tooth is traversed by ridges which have generally a transverse direction to the long axis of the jaw in which the tooth lies. The ridges may be regarded as having been developed between tubercles which they connect and whose distinctness as tubercles is thereby destroyed. Lophodont teeth are only found in vegetable-feeding animals.
Fig. 36.—Molar teeth of Aceratherium platycephalum. × ½. m.1-m.3., Molars; mh, metaloph; p.1-p.4, premolars; ph, protoloph; ps.f, parastyle fossa; te, tetartocone. (After Osborn.)
The special characteristics of the teeth of various groups of animals will be considered further under the accounts of the several orders of recent and fossil Mammalia.
A very general feature of the teeth of the Mammalia is what is usually termed the diphyodont dentition. In the majority of cases there are two sets of teeth developed, of which the first lasts for a comparatively short time, and is termed on account of its usual time of appearance the "milk dentition"; this is replaced later by the permanent dentition. In lower vertebrates the teeth are replaced as worn away. There is not, however, so great an antithesis in this matter between the Mammalia
