[Illustration: Plate VI.--The series of teeth in the upper (1) and lower jaw (2) of a modern European (natural size). The teeth are placed closely side by side without a gap--an arrangement which does not occur in the apes nor in any other living mammal, although it is found in some extinct herbivores--the Anoplotherium and the Arsinoeitherium. The shape of the arch formed by the row of teeth should be compared with that shown by the same arch in the Gibbon (Pl. VII). The crowns of the teeth are very carefully drawn in this figure, which is from a plate published by Professor Selenka. It must be noted that the number of tubercles on the true molars may be in exceptional cases one more or one less than that given in this drawing which gives the most usual number. The word "molar" is often used to include the five cheek-teeth on each side of each jaw, but more strictly the anterior bicuspid teeth are called "pre-molars," and the three larger teeth behind them, which have no predecessors or representatives in the first or milk dentition, are called true molars or simply "molars"--a rule we have followed here. In both upper and lower jaw we see the four incisors in the middle (Inc. 1, Inc. 2); on each side of them is the conical crown of a canine--a tooth which is greatly enlarged in the ape (see Pl. VII), but is no larger proportionately than it is here even in the most ancient known human jaw, that from the Pleistocene of Heidelberg (see "Science from an Easy Chair," Methuen, 1910, p. 405). The two small bicuspid "pre-molars" and the three large molars follow these on each side in each jaw. The crown of the most anterior (or "first") molar of the upper jaw has four cusps, tubercles, or cones on it. It is "quadri-tuberculate." The second and third molars of the upper jaw have three such prominent tubercles (excluding a row of small tubercles on the hinder margin of the second); they are, in fact, tri-tuberculate; whilst the two hindermost molars of the lower jaw have four tubercles and are called quadri-tuberculate. The first molar (M1) of the lower jaw has in this specimen five tubercles. In 60 per cent. of European lower jaws this is the case. But in 40 per cent. this tooth is quadri-tuberculate. In Polynesians, Chinese, Melanesians and negroes five tubercles are found on this tooth in 90 per cent. of the jaws examined. The apes are characterised by five tubercles on this tooth, and they are found also on the first lower molars of prehistoric men. Four tubercles only on this tooth is a departure from the ape's condition and is found more frequently in Europeans.