the chemical action of the food, &c., constituting what are called the "marks" of the mouth; and as the teeth undergo wear, these marks are ultimately lost. The period at which they may disappear, and the appearances which they may put on, are well shown in a plate which Mr. Fores has published, entitled "_The Age of the Horse_," and to that plate I therefore refer the reader; but, while doing so, I must caution him not to depend too much upon a sign, which is only true as a general rule; for as the infundibula in all animals are not of the same depth, or supposing them to be of one depth, the crusta petrosa is seldom of the like thickness in any two horses, and the teeth do not wear uniformly in different subjects, of course the indications they exhibit cannot be absolutely relied upon. Those indications, however, ought to be known, and should never be disregarded. Some animals of seventeen years of age, and even in rare instances of twenty-eight years old, will retain the marks in two or more of the incisors; nevertheless, as a general rule, these signs are worthy of attention, and, taken in conjunction with the illustrations presented in this work, afford all that can be obtained to confirm the judgment. 17. _There are infundibula also in the molar teeth_, but not in all of them. Those of the lower jaw have no infundibula, but only grooves or fissures, into which the crusta petrosa dips, and which it fills up. The manner of the arrangement will be seen in the table of one of these teeth represented in figure 2, page 26. In the same place, the wood-cut, figure 1, depicts the grinding surface of one of the superior molars. Every tooth of the last description, possesses two infundibula, which extend almost to the root, and are possessed of the same coverings as the like cavities in the incisors. The crusta petrosa, however, which lines the infundibula of the superior molars, is much thicker than the similar membrane inverting the like parts in the incisors; consequently, absolute cavities, answering to the marks of the nippers, are of comparatively slight extent, soon obliterated, and not generally found in any of these teeth after the horse is six years old; indeed all the marks are often obliterated before the fifth year.