The outer surface is convex and furnished with a scarcely apparent central longitudinal ridge. On the inner surface, or that which is turned to the cavity of the mouth, there are from two to three longitudinal furrows reaching from the neck to the centre of the tooth. The lower teeth are of a three-sided, pyramidal form, presenting an interior, posterior, and inner superficies. The pre-molars of an aged male gorilla are wide, and are furnished with a large outer, and a smaller inner, cusp. The three four-cusped upper molars display a more regular and symmetrical arrangement of their cusps than is the case with the female, in which the position of the cusps is rather variable. Except for the difference of size, the relative conditions of these teeth are the same in male and female. The first pointed lower pre-molars are in the male of the form of a four-sided pyramid, convex on the anterior and outer surface, flat on the side directed to the cavity of the mouth, and marked with furrows on the posterior surface. The small second and lower pre-molars have two anterior and one posterior cusp. The last is generally worn away at an early age. Each molar tooth has two outer and two inner cusps, opposite to each other, and one posterior cusp. We cannot here fail to notice the likeness to the conditions of the human teeth, a likeness which is still more striking in the female. In the chimpanzee, also, the upper central incisor teeth are broadly chisel-shaped, while the upper and lower lateral incisors are smaller. In the male there is often a considerable gap between these and the canine teeth. The latter present the form of a three-sided pyramid, of which the anterior edge is blunt and tends outwards, while the posterior angle is sharp, scooped out in its upper third, and terminating at the base of the crown in a posterior cusp. The pre-molars have an external and an inner cusp; the molars have two external and two inner cusps, connected with each other by their enamel. The lower canine teeth of these animals are likewise of the shape of a three-sided pyramid, of which the anterior angle is very blunt, while the inner and posterior angles are sharply cut. The anterior surface is not grooved like the upper canine teeth. The lateral angle is much rounded. The back teeth plainly display the posterior fifth cusp, which may also be observed in man. In the orang-utan the characteristics of the upper incisors are such as we have described in the case of other anthropoids. The upper canine teeth are shaped like a three-sided pyramid, and are furnished with a longitudinal furrow on the anterior side. A similar furrow is found on the posterior superficies of the lower canine teeth. The back teeth display no special characteristics when compared with those of other anthropoids.