Fig. 6 is a milk or caduct tooth, and Fig. 7 represents the canine or tusks, which are missing in mares, or, if they exist, are only rudimentary.

The incisors implanted in the two maxillary bones form what is known as the incisive arch. Meeting naturally those of the upper with those of the lower jaw they form two arches or curves—one from above below, and one from one side to the other, superoinferior and transversal arch, which vary much, ac

cording to the age of the animal and specially in the old ones.

A tooth is said to be worn when the external dental cavity has been much reduced in depth by wearing of the plate, and when there remains but the cul de sac of the same cavity surrounded by central enamel.

The life of an animal may be divided, taking in consideration the changes that take place in the evolution and alteration of the teeth, into three periods:

1st.-Change of caduct teeth.

2d.-Growth and wearing of the adult. 3d.-Change in the adult.

FIRST PERIOD.

Colts are generally born in the Spring-no incisives present. First and second molar, and at the end of the first month, the third molar. The incisives, however, will make their appearance in from twenty-four hours to a few days. The nippers

then come out by their anterior border, and at one month (Fig. 8) the posterior border is on a level.

About the same time the dividers begin to show themselves, and for some time the colt remains with eight incisives only, the growth taking place between thirty and forty days (Fig. 9).

The corners vary in their growth between the sixth and tenth months (Fig. 10).

All these teeth, as they gradually appear, undergo a process of wearing, and in this way we obtain another character, viz.: that the caduct nippers are worn at about ten months to a year, the dividers are at eighteen months, and at two years a full mouth of worn milk teeth is found.

SECOND PERIOD.

2 to 31. Appearance and growth of the caduct nippers; milk dividers and corner teeth worn (Fig. 11).

3 to 4. The nippers are those of second dentition. The permanent dividers make their appearance and grow out of the alveoli. Milk corner teeth worn loose, more or less, and ready to make room for the corners of second dentition (Fig. 12).