The true molars (fig. 586., in. 1, in. 2, m. 3) are quadricuspid, relatively larger in comparison with the bicuspids than in the Orang. In the first and second molars of both species of Chimpanzee a low ridge connects the antero internal with the postero•external cusp, cross ing the crown obliquely, as in Man. There is a feeble indication of the same ridge in the unworn molars of the Orang ; but the four principal cusps are much less distinct, and the whole grinding surface is flatter and more wrinkled than in the Chimpanzee. In the Troglodytes niger the last molar is the smallest, owing to the inferior development of the two hinder cusps, and the oblique connecting ridge is feebly marked. In the Troglodytes Gorilla this ridge is as well developed as in the other molars, but is more transverse in position ; and the crown of rn. 3 is equal in size to that of' nz. 1 or tn. 2, having the pos terior outer cusp, and particularly the pos terior inner cusp, more distinctly developed than in the Troglodytes niger. The repe tition of the strong sigmoid curves which the unworn prominences of the first and se cond true molars present in Man, is a very significant indication of the near affinity of the Chimpanzee as compared with the ap proach made by the Orangs or any of the inferior Quadrumana, in which the four cusps of the true molars rise distinct and indepen dently of each other. A low ridge girts the base of the antero-internal cusp of each of the upper true molars in the male Chimpan zee: it is less marked in the female. The premolars as well as molars are severally im planted by one internal and two external fangs, diverging but curving towards each other at their ends as if grasping the substance of the jaw. I have found the two outer fangs of the second premolar connate in one female specimen of the Troglodytes niger. In no variety of the human species are the premo lars normally implanted by three fangs ; at most the root is bifid, and the outer and inner divisions of the root are commonly connate. It is only in the black varieties, and more particularly that race inhabiting Australia, that I have found the wisdom tooth, in. 3, with three fangs as a general rule ; and the two outer ones are more or less confluent.
In the lower jaw of the great Chimpanzee the lateral incisors are broader than the middle ones, although they are smaller rela tively than in the Troglodytes niger; they are larger and less vertically- implanted than in Man. The lower canines are two inches and a half in length, including the root ; the enamelled crown is an inch and a quarter in length, and nearly an inch across the base; it is conical and trihedral ; the outer and ante rior surface is convex, the other two surfaces are flattened or subconcave, and converging to an almost trenchant edge directed inwards and backwards; a ridge separates the convex from the antero-internal flat surface; both this and the posterior surface show slight traces of a longitudinal rising at their middle part. The lower canine of the male shows the same relative superiority of size as the upper one compared with that in the female in both species of Chimpanzee. The canine almost touches the incisor, but is separated by a diasterna one line and a half broad from the first premolar. This tooth (p. 3) is larger externally than the second premolar, and is three times the size of the human first premolar (p. 3); it has a subtrihedral crown, with the anterior and outer angle produced forwards, slightly indicating the peculiar fea ture of the same tooth in the Baboons, but in a less degree than in the Orang. The summit of the crown of p. 3 terminates in two sharp trihedral cusps, the outer one rising highest, and the second cusp being feebly indicated on the ridge extending from the inner side of the first ; the crown has, also, a thick ridge at the inner and posterior part of its base.
The second premolar (p.4) has a subquad rate crown, with the two cusps developed from its anterior half, and a third smaller one from the inner angle of the posterior ridge. Each lower premolar is implanted by two antero-posteriorly compressed divergent fangs, one in front of the other, the anterior fang being the largest. The three true molars are equal in size in the Troglodytes Gorilla ; in the' Troglodytes niger (fig. 588.) the first (m. 1) is a little larger than the last (in. 3), which is the only molar in the smaller Chimpanzee as large as the corresponding tooth in the black varieties of the human subject *, in most of which, especially the Australians, the true molars attain larger dimensions than in the yellow or white races. The four principal cusps, especially the two inner ones, of the first molar of both species of Chimpanzee are more pointed and prolonged than in Man ; a fifth small cusp is developed behind the outer pair, as in the Orangs and the Gibbons, but is less than that in Man. The same additional cusp is present in the second molar, which is seldom seen in Man. The crucial groove on the grinding surface is much less distinct than in Man, not being continued across the ridge connecting the anterior pair of cusps in the Chimpanzee. The crown of the third molar is longer antero-posteriorly from the greater development of the fifth posterior cusp, which, however, is rudimental in comparison with that in the Semnopitheques and Macaques. All the three true molars are supported by two distinct and well developed antero-pos teriorly compressed divergent fangs, longitu dinally excavated on the sides turned towards each other ; in the white and yellow races of the human subject these fangs are usually connate in in. 3, and sometimes also in in. 2. The molar series in both species of Chim panzee forms a straight line, with a slight ten dency in the upper jaw to bend in the opposite direction to the well-marked curve which the same series describes in the human subject.
This difference of arrangement, with the more complex implantation of the premolars, the proportionally larger size of the incisors as compared with the molars ; the still greater relative magnitude of the canines ; and, above all, the sexual distinction in that respect, illus trated by figs. 585. 587., stamp the Chimpan zees most decisively with not merely specific but generic distinctive characters as compared with Man. For the teeth are fashioned in their shape and proportions in the dark re cesses of their closed formative alveoli, and do not come into the sphere of operation of external modifying causes until the full size of the crowns has been acquired. The formid able natural weapons, with which the Creator has armed the powerful males of both species of Chimpanzee, form the compensation for the want of that psychical capacity to forge destructive instruments which has been re served as the exclusive prerogative of Man. Both Chimpanzees and Orangs differ from the human subject in the order of the de velopment of the permanent series of teeth ; the second molar (ni. 2) comes into place before either of the premolars has cut the gum, and the last molar (m. 3) is acquired before the canine. We may well suppose that the larger grinders are earlier required by the frugivorous Chimpanzees and Orangs than by the higher organised omnivorous spe cies with more numerous and varied resources, and probably one main condition of the earlier development of the canines and premolars in Man may he their smaller relative size.