transverse arch of the teeth becomes narrower (Fig. 19).

15 years. The nippers are triangular, the dividers becoming so, the corners still rounded.

16 years. The dividers are nearly triangular and the corner teeth assuming that shape.

17 years. The corners are still more so (Fig. 20), and at 18 years the incisive arch is occupied with triangular teeth all through (Fig. 21).

19 years. The last change begins. The nippers assume a flattened sideways shape. At

20 years they are flattened from side to side, the dividers becoming so. At

21 years these last are also flattened sideways, when at

22 years the entire arch contains also teeth flattened in the same conditions, representing the last

form represented by the tooth towards its root (Fig. 22.)

From 19 years the horizontality of the teeth is becoming more and more marked in such a manner that at 21 the inferior incisive arch is almost straight.

IRREGULAR MOUTHS.

On account of the peculiar nature of some of the elements of the tooth and of its anatomical structure, some jaws will either wear out too rapidly, while others will not, and on this account a young

animal may present an old vidual will show all the

mouth, or an old indicharacters of a young

mouth. To approximate the age exhibited by such a mouth we will take off from the characters shown in the first case, or add to those of the second as many years as by approximation the tooth will show in measuring by lines the number which would be necessary to add or to take off to give the tooth its normal length.

Fig. 23 shows a defective jaw of an old horse, in

which the external dental cavity is yet remaining; the excessive length of the teeth allows us to make out the age by adding to the age given by the marks of the crown as many years as there are lines in excess in its length.