The surest indication of the age is given by the teeth. The calf, at birth, will usually have two incisor or front teeth; in some cases just appearing through the gums; in others, fully set, varying as the cow falls short or exceeds her regular time of calving. If she overruns several days, the teeth will have set and attained considerable size, as appears in Fig. 15. During the second week, a tooth will usually be added on each side, and the mouth will generally appear as in Fig. 16; and, before the end of the third week, the animal will generally have six incisor teeth, as shown in Fig. 17; and in a week from that time the full number of incisors will have appeared, as seen in Fig. 18. [Illustration: Fig. 17. Third week.] [Illustration: Fig. 18. Month.] These teeth are temporary, and are often called milk-teeth. Their edge is very sharp; and, as the animal begins to live upon more solid food, this edge becomes worn, showing the bony part of the tooth beneath, and indicates, with considerable precision, the length of time they have been used. The centre or oldest teeth show the marks of age first, and often become somewhat worn before the corner teeth appear. At eight weeks, the four inner teeth are nearly as sharp as before. They appear worn not so much on the outer edge or line of the tooth, as inside this line; but, after this, the edge begins gradually to lose its sharpness, and to present a more flattened surface; while the next outer teeth wear down like the four central ones; and at three months this wearing off is very apparent, till at four months all the incisor teeth appear worn, but the inner ones the most. Now the teeth begin slowly to diminish in size by a kind of contraction, as well as wearing down, and the distance apart becomes more and more apparent. [Illustration: Fig. 19. Five to eight months.] [Illustration: Fig. 20. Ten months.] [Illustration: Fig. 21. Twelve months.] [Illustration: Fig. 22. Fifteen months.] From the fifth to the eighth month the inner teeth will usually appear as in Fig. 19; and at ten months this change shows more clearly, as in Fig. 20, and the spaces between them begin to show very plainly, till at a year old they ordinarily present the appearance of Fig. 21; and at the age of fifteen months that shown in Fig. 22, where the corner teeth are not more than half the original size, and the centre ones still smaller. [Illustration: Fig. 23. Eighteen months.] [Illustration: Fig. 24. Two years past.]