The structure of the cheek teeth is very diverse among the Mammalia. Broadly, two types are to be recognised. There are {51} 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. [Illustration] FIG. 36.--Molar teeth of _Aceratherium platycephalum_. x 1/2. _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. [Illustration] FIG. 37.--Two stages in the development of the teeth of a Mammal (diagrammatic sections). _alv_, Bone of alveolus; _dent_, dentine; _dent.s_, dental sac; _en_, enamel; _en.m_, enamel membrane; _en.m_^2, enamel membrane of permanent tooth; _en.plp_, enamel pulp; _gr_, dental groove; _lam_, dental lamina; _lam_', part of dental lamina which grows downwards below the tooth germ; _n_, neck connecting germs of milk and permanent tooth; _pap_, dental papilla; _pap_^2, dental papilla of permanent tooth. (After O. Hertwig.)