mination of the exact homologies of the teeth, it is satisfactory to know that the more constant and important character of develop ment gives the requisite certitude as to the nature of the so-called bicuspids in the human subject. In fig. 584., the condition of the teeth is shown in the jaws of a child of about six years of age. The two incisors on each side (di.) are followed by a canine, dc., and this by three teeth having crowns resembling those of the three molar teeth of the adult. In fact, the last of the three is the first of the permanent molars ; it has pushed through the gum, like the two molars which are in advance of it, without displacing any previous tooth, and the substance of the jaw contains no germ of any tooth destined to displace it : it is therefore, by this character of its de velopment, a true molar, and the germs of the permanent teeth, which are exposed in the substance of the jaw between the diverging fangs of the molars, d. 3 and d. 4, prove those molars to he temporary, destined to be replaced, and prove also that the teeth about to displace them are premolars. According, therefore, to the rule previously laid down, we count the permanent molar in place the first of its series (am. 1), and the adjoining premolar as the last of its series, and consequently the fourth of the typical dentition, or p. 4.
We are thus enabled, with the same scien tific certainty as that whereby we recognise in the middle toe of the foot the homologue of that great digit which forms the whole foot and is encased by the hoof in the horse, to point to p. 4, or the second bicuspid in the upper jaw, and to nt. I, or the first molar in the lower jaw of man, as the homo logues of the great carnassial teeth of the lion and tiger. We also conclude that the teeth which are wanting in man to complete the typical molar series, are the first and second premolars, the homologues of those marked p. 1 and p. 2 in the bear. The characteristic shortening of the maxillary bones required this diminution of the number of their teeth, as well as of their size, and of the canines more especially ; and the still greater curtailment of the premaxillary bone is attended with a diminished number and an altered position of the incisors. One sees, indeed, in the car nivorous series, that a corresponding decrease in the number of the premolars is concomitant with the shortening of the jaws. Already in the Mustelidee, (fig. 580., IV), p. 1 below is abrogated; in Felix also above, with the further loss of p. 2 in the lower jaw ; the true molars being correspondingly reduced in these strictly flesh-eating animals, but taken away from the back part of their series.
If we were desirous of further testing the soundness of the foregoing conclusions as to the nature of the teeth absent in the reduced dental formula of man, we ought to trace the mode in which the type is progressively resumed in descending from man through the order most nearly allied to our own.
Through a considerable part of the Qua drumanous series, e. g. in all the Old World genera above the Lemurs, the same number and kinds of teeth are present as in man ; the first deviation being the disproportionate size of the canines and the concomitant break or " diastema " in the dental series for the reception of their crowns when the mouth is shut. This is manifested in both the Chimpanzees and Orangs, together with a sexual difference in the proportions of the canine teeth.
As the precise characteristics of the human dentition are best demonstrated by comparison with that brute species which is most nearly allied to Man, and makes the first step in the descending scale, I here subjoin the details of such a comparison, which is the more required since it is not touched upon in the article Qu ADRUMANA, and will be the more acceptable as one of its subjects is a species of Chim panzee (Troglodytes Gorilla)*, unknown to science when that article was written, and which, so far as its organisation is known, is more anthropoid than even the docile and smaller species of Chimpanzee (Troglodytes niger). A side view of the teeth of a male,
full-grown, but not aged, specimen of the great Chimpanzee is given of the natural size in fig. 585., and a view of the working surface of the whole series of the tipper jaw in fig. 586. This dentition, though in all its principal characters strictly quadrumanous, yet, in the minor particulars in which it differs from the dentition of the Orang, approaches nearer the human type. In the upper jaw the middle incisors (fig. 586, i. 1) are smaller, the lateral ones (ib. i. 2) larger than those of the Orang e; they are thus more nearly equal to each other; nevertheless the proportional superiority of the middle pair is much greater than in Man, and the proportional size of the four incisors both to the entire skull and to the other teeth is greater. Each incisor has a prominent posterior basal ridge, and the outer angle of the lateral incisors, i. 2, is rounded off as in the Orang. The incisors incline forwards from the vertical line as much as in the great Orang. Thus the characteristics of the human incisors are, in addition to their true incisive wedge-like form, their near equality teeth "*, when the mouth is closed, is appli cable only to the female, and does not distin guish the Chimpanzees from the Orangs. In the male of the smaller Chimpanzee (73•oglody. tea ?tiger) the upper canine is conical, pointed, but more compressed than in the Orang, and with a sharper posterior edge ; convex ante riorly, becoming flatter at the posterior half of the outer surface, and concave on the corre sponding part of the inner surface, which is traversed by a shallow longitudinal impres sion ; a feeble longitudinal rising and a second linear impression divide this from the convex anterior surface, which also bears a longitu dinal groove at the base of the crown. The canine is rather more than twice the size of that in the female. In the male Gorilla (figs. 585, 586.), the crown of the canine is of size, their vertical or nearly vertical posi tion, and small relative size to the other teeth and to the entire skull. The diastema be tween the incisors and the canine on each side is as well marked in the male Chim panzee as in the male Orang.t The crown of the canine (ib. c.), passing outside the in terspace between the lower canine and pre molar, extends in the male Troglodytes Gorilla a little below the alveolar border of the under jaw when the mouth is shut ; the upper ca nine of the male Troglodytes niger likewise projects a little below that border ; the seventh character, therefore, which I had formerly assigned to the genus, "apices of canines lodged in intervals of the opposite more inclined outwards : the anterior groove on the inner surface of the crown is deeper ; the posterior groove is continued lower down upon the fang, and the ridge between the two grooves is more prominent than in the Troglodytes niger. Both premolars (fig. 586. p. 3 and p. 4) are bicuspid ; the outer cusp of the first and the inner cusp of the second being the largest, and the first premolar consequently appearing the largest on an external view (fig. 585.). The difference is well marked in the female (fig. 587, p. 3, p. 4). The anterior ex ternal angle of the first premolar is not pro duced as in the Orang, which in this respect makes a marked approach to the lower Qua drumuana. In Man, where the outer curve of the premolar part of the dental series is greater than the inner one, the outer cusps of both premolars are the largest ; the alternating su periority of size in the Chimpanzee accords with, and contributes to, the straight line which the canine And premolars form with the true molars.