These have their crowns flattened from above downwards, and gradually become thinner from the root to the anterior border, which is edged and slightly convex. These teeth gradually wear away. In proportion to the progress of this wear, on account of the fact that it involves the anterior borders and upper surfaces of the incisor teeth, and that these teeth are narrower towards the root than at the opposite extremity, the intervals which separate them tend to become wider and wider; and when the roots become exposed by the retraction of the gums, they are then separated from one another by a considerable interval. The molars have their grinding surface comparable to that of the horse; they increase in size from the first to the sixth. The teeth of the horse are forty in number; they are thus distributed: 6_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 6_m._ --------------------+-------------------- = 40. 6_m._ 1_c._ 3_i._ | 3_i._ 1_c._ 6_m._ As they become worn, these teeth continue to grow, and as, on the one hand, this phenomenon takes place throughout the whole life of the animal, and, on the other hand, the process of wear brings out and makes visible at the surface of friction parts formerly deeper and deeper, and of which the configuration varies at different levels, there result special features which permit the determination of the age of the animal by an examination of its jaws. The incisors are called, commencing with those situated nearest the middle line, _central incisors_ or _nippers_, _intermediate_ and _corner incisors_. The canines, also designated as the _fangs_, exist only in the male. It is exceptional to find them in the mare, and when they exist in this latter they are less developed than those of the horse. The molars have cuboid crowns; the surface of friction is almost square in the case of the upper molars, and is inclined so as to look inwards; in the case of the inferior ones, it is a little narrowed, and is inclined so as to look outwards. In the upper jaw the external surface of the crown is hollowed by two longitudinal furrows; in the lower jaw the same surface has only one furrow, which at times is but slightly marked. In the hare the teeth are twenty-eight in number: 6_m._ 0_c._ 2_i._ | 2_i._ 0_c._ 6_m._ --------------------+-------------------- = 28. 5_m._ 0_c._ 1_i._ | 1_i._ 0_c._ 5_m._ The four incisors of the upper jaw are divided into two groups; one of these is formed by the two principal teeth, the other by two very small incisors which are placed behind the preceding.