FIG. 13.—Crown of a Grinder of the Ox. _a_, cement; _b_, enamel; _c_, dentine; _d_, enamel; _e_, cement. ] There are also great differences in the way of resting the feet on the ground. Man and bear tread on the soles of the feet (plantigrade); dog and cat walk on the under side of the toes (digitigrade), not on the other parts of the feet. Ox, pig, horse, etc., rest while walking only on the tips of the toes, which are sheathed in hoofs (unguligrade). The teeth of mammals are wedged into special sockets in the jaw-bones. The structure of a mammalian tooth is made clear by Fig. 12. We first distinguish a pulp-cavity (_p_), which in the living animal is filled with a substance supplied by a bloodvessel and nerve. This cavity is surrounded by the dentine (_d_), a hard substance which makes up the greater part of the tooth. Hard enamel (_s_) covers the whole of the crown in man and many animals, while in certain other forms it is found only on part of the crown. The root of the tooth is covered with cement (_z_), a bone-like substance. All teeth in which the entire surface of the crown is covered by enamel only are known as _simple teeth_, while those into which the enamel only penetrates in more or less deep folds, leaving the rest of the crown uncovered, are known as _compound teeth_ (Fig. 13). The structure of the teeth is related to the nature of the food. We distinguish three kinds of teeth in the same animal, which, however, are not all present in every species; these are the _incisors_, _canines_, and _grinders_. The first two kinds are changed; but only the anterior grinders, known as the _premolars_, are changed, while the hinder ones, the _true molars_, do not first appear as “milk” teeth, but rather later on with the other “permanent” teeth. The following orders of Mammals are distinguished: I. Bimana (Man), II. Quadrumana (Apes), III. Carnivora (Beasts of prey), IV. Insectivora (Insect-eaters), V. Cheiroptera (Bats), VI. Rodentia (Gnawers), VII. Ruminantia, VIII. Solidungula, IX. Pachydermata, X. Cetacea, XI. Edentata, XII. Marsupialia (Pouched animals), XIII. Monotremata. I shall deal here only with those orders which are of agricultural importance. ORDER: =Carnivora= (BEASTS OF PREY).