The third upper molar (m 3) is the only tooth wanting to complete the typical heterodont mammalian dentition.
In some all the teeth are nearly of the same size; others possess in front of the jaws (Lycodonts) or behind in the maxillaries (Diacrasterians) a tooth more or less con spicuously larger than the rest; whilst others again are distinguished by this larger posterior tooth being grooved along its outer face.
Finally, we have the family Rhinocerotidae, which includes the existing representatives of the group. In this family the dentition has undergone considerable reduction, and may be represented inclusive of all the variations, by the formula i a or a m a The first upper incisor, whenpresent, has an 430r2; PP antero-posteriorly elongated crown, but the second is small; when fully developed, the lower canine is a large forwardly directed tusk-like tooth with sharp cutting-edges, and biting against the first upper incisor.
The figure that now stood by its bows was tall and swart, with one white tooth evilly protruding from its steel-like lips.
Let BB be part of the pitch-circle, and a the point where a tooth is to cross it.
Of these the first upper premolar is a simple tooth placed close behind the premaxilla and separated by a long gap from the two other teeth of the same series; while the lower incisors, of which the outermost is the largest, are directed partially forwards.
In the male the right tooth usually remains similarly concealed, but the left is immensely developed, attaining a length equal to more than half that of the entire animal.
This distinction is, however, not so important as it appears at first sight, for their connexion with the bone is only of a secondary nature, and, although it happens conveniently that in the great majority of cases the division between the bones coincides with the interspace between the third and fourth tooth of the series, still, when it does not, as in the mole, too much weight must not be given to this fact, if it contravenes other reasons for determining the homologies of the teeth.
This independence of adaptation applies to every detail of structure; the six cusps of a grinding tooth may all evolve alike, or each may evolve independently and differently.
If the same rolling curve R, with the same tracing-point T, be rolled on the outside of any other pitch-circle, it will have the fare of a tooth suitable to work with the flank AT.