These three teeth form a series of intermediate steps between a most ancient molar and the modern molar of the human type. The second tooth is halfway between the first and third. The second molar, seen from above, has exactly the same cusps as the first, so it is not difficult to recognise that each cusp has been directly derived from its fellow. The third tooth of the series (Fig. 7) has lost one of its cusps; it has lost a cusp of the triangle. It is now a tooth where only half the triangle is left on the anterior side and with a very long heel. That tooth has exactly the same pattern as the lower human molar tooth (Fig. 8), the only difference is that the heel is somewhat more prolonged. These teeth belong to one of the oldest fossil monkeys, anaptomorphus. Human lower molars, not very exceptionally, instead of four cusps, have five. The fifth cusp always appears in the middle of the heel, or between the posterior lingual and the posterior buccal. This occurs in monkeys and other animals, but no record exists of the ancient anterior lingual reappearing. The human lower molar, with its low, quadritubercular crown, has hence evolved by addition of cusps and by gradual modelling from a high-crowned, simple, pointed tooth. Human teeth are of excellent service in the initial determination of degeneracy in the child. For this purpose the teeth should be studied from the first evidence of their development until they are all in place, which occurs normally, in most cases, by the twenty-second year.