ALIMENTARY APPARATUS. The dental formula common to the Solipeda is as follows :— Incisors canine premolars, 3_3, molars, 3_3 The canine teeth, however, it must be observed, only exist in the male sex.
The incisor teeth, in the generality of her bivorous quadrupeds, are bevelled off' paste riorly, so as to. present in front chisel-like cutting edges ; but in the Solipeds, when pung, the lateral incisors are furnished with two cutting edges, one in front and the other behind, from which circumstance those cen tral fossm are produced which, as we shall see further on, furnish important testimony relative to the age of the animal.
The canine teeth, here called "tusks," or " lushes," are always of very moderate dimen sions, and their points, at an early age, become flattened and blunt. Those of the upper jaw are separated from the incisors by a con siderable interval ; and a similar interspace also exists, but to a less extent, in the lower jaw.
The molar teeth of the horse are of a pris matic form, their grinding surfaces being marked with four crescents of enamel in the lower jaw, and with five in the tipper : these crescentic patches in the upper jaw have their concavities turned outwards, but in the lower jaw in the opposite direction. The teeth of the horse are, moreover, distinguishable from those of the ox and some other Ituminants, which they resemble in their general appear ance, from the circumstance that, in the latter, the crescentic patches of enamel are arranged in pairs, and are placed parallel to each other ; whilst in the horse they are situated alter nately, the first of the inner margin of the tooth corresponding to the interval between the two of the outer margin.
Professor Owen*. observes, that the cha racter by which the horse's molars may be best distinguished from the teeth of other Herbivora corresponding with them in size, is the great length of the tooth before it divides into fangs. This division, indeed, does not begin to take place until much of the crown has been worn away ; and thus, except in old horses, a considerable portion of the whole of the molar is implanted in the socket by an undivided base. The deciduous molars have shorter bodies, and sooner begin to develope roots ; but in these, or in an old permanent molar with roots, the pattern of the grinding surface, though it be a little changed by partial obliteration of the enamel folds, yet generally retains as much of its cha racter as to serve, with the form of the tooth, to distinguish such tooth from the permanent molar of a Ruminant.
A knowledge of the structure and history of the teeth of the horse becomes addition ally important, from the circumstance that it is from the condition of the dental apparatus that an estimate may be formed concerning the age of the animal ; and, in order to un derstand the data thus afforded, it will be necessary to consider the structure of these organs rather more closely.
The incisors ±, when the permanent teeth are first completely developed, are arranged close together, forming the arc of a circle at the extremity of both jaws ; they are slightly curved, with long simple sub-trihedral fangs, tapering to their extremity. The crowns are broad, thick, and short; the contour of the biting surface, before it is much worn, ap proaching an ellipse. These teeth, if found detached, recent, or fossil, are distinguishable from those of tbe Ruminants by their greater curvature, and from those of all other animals by a fold of enamel, which penetrates the body of the crown front its broad flat summit, like the inverted finger of a glove. When the tooth begins to be worn, the fold forms an island of enamel, inclosing a cavity partly filled with cement, and partly by the dis coloured substances of the food, and is called " the nzark." In aged horses the incisors are worn down below the extent of the fold, and " the mark " disappears. The cavity is usu ally obliterated in the first, or mid-incisors, at the sixth year ; in the second incisors at the seventh year, and in the third, or outer incisors, in the eighth year, in the lower jaw. It remains longer in those of the upper jaw, and in both the place of the " mark " con tinues for some years to be indicated by the dark-coloured cement, even to about sixteen years old. At this period the worn sumrnits of the incisors present a subtriangular form.