Such may be regarded as the special characteristics of these three kinds of teeth, nevertheless there seems to be a common type embracing all, and there are transitional forms. The incisors are sometimes conical, like the canines this is very obvious in the great polar bear, as well as certain other carnivora; and in the elephant, the same teeth constitute the tusks. In the insect-eating bat, the projections of the molars are conical; in the dolphin and others of the whale order, all the teeth are conical. There appear, therefore, to be clear indications that the cone may be considered the typical form of tooth.

As is remarked, however, by Professor Owen, "shape and size are the least constant of dental characters in the mammalia, and the homologous teeth are determined, like other parts, by their relative position, by their connexions, and by their development." We have already stated by what mark, founded on position, the incisors may be invariably distinguished, and that the canines succeed them in order from before backwards. The mode of development of the remaining teeth enables us to determine their homologies. In most mammalia there are two sets of teeth, the milk or deciduous, and those which succeed them, called permanent. Now, some of the milk-molars are directly succeeded by permanent teeth, which displace them vertically; others, which appear as the animal advances in age, do not displace predecessors; and hence a distinction of the molars into premolars or false molars, and true molars. The second set of premolars occupy the place of others which preceded them; the true molars are situated more posteriorly, and have had no prede

cessors.

Investigations of anatomists have shewn that, in re

ference to number, the teeth follow a type, and the typical dental formula is thus indicated:

3-3
3-3'

1-1

Incisors, canines, premolars, molars, 33

The figures above and below the lines express the respective numbers of teeth on each side in upper and lower jaw.

A common plan also prevails in the general structure of the teeth. Three distinct substances, differing in hardness and in microscopical characters, enter into their formation. A perfect mammalian tooth may be considered as formed principally of hard dentine or ivory in the centre, still harder enamel on the crown, and most external of all, a layer of cement, which is softer than the other two.

FIG. 44. Longitudinal section of human incisor, magnified p, pulp cavity; d, dentine or ivory; e, enamel; c, cement.

FIG. 44.

SECT. II. TRACES OF SPECIAL ADAPTATION IN THE NUMBER, FORM, AND STRUCTURE OF TEETH.

In these organs, of so much importance in the animal economy, and of such scientific interest to the zoologist and palæontologist, we find numerous examples of modifications in respect of structure, form, and number, all for the fulfilling of useful ends.

Remarkable complex modifications occur when the substances of which the teeth consist, instead of being simply superposed, and shewing a comparatively regular surface, present infoldings of the surface, and consequent interblending or apparent mixture of the dentine, enamel, and cement. It is obvious that in all cases in which there is interblending of parts differing in hardness, by whatever means effected, a rough surface is kept up by their being unequally worn away; a most admirable, and, at the same time, simple contrivance for forming an effectual triturating surface, on the same principle that a millstone must be rough in order to perform its office fully.

There are numerous and remarkable special modifications of teeth, all of which are so obviously in harmony with the particular purpose they are intended to serve, that no one disputes the propriety of adducing such in illustration of final cause. These have been discussed with considerable fulness by different writers on natural theology, and it is less necessary to enter into details here; a few of the more prominent examples may suffice, and we must here acknowledge the valuable contribution to this department of the history of the teeth embodied in the treatise of the Hunterian Professor.1

Among bats we find three varieties in the kind of food; some subsist on insects which they capture during flight; others feed on soft vegetable food, such as fruits; and a few suck the blood of animals, whose skin they puncture during sleep. The insectivorous bats have all the teeth characterized by the presence of numerous sharp conical points well fitted for capturing and retaining their living prey. In frugivorous species the molars are well developed, and are suited by their blunt tuber

1 Owen's Odontography.

cular crowns to reduce the vegetable matter to the state of pulp. In true bloodsucking species, the vampyres, the upper incisor teeth, and canines, are well fitted to puncture the skin, while the molars are deficient in number, the blood on which they feed requiring no trituration.

The Rodentia, or gnawing animals, (rat, hare, &c.,) present a peculiar arrangement (one of their leading characters) in the front or incisor teeth, which are in continual requisition for gnawing or cutting hard vegetable matter. The front part of the incisors, and it only, has a thick layer of enamel, the consequence of which arrangement is, that the softer dentine behind being continually worn away, a sharp chisel-shaped edge is always kept up. But since both, though thus unequally, are also liable to wear away, an arrangement is added in no less beautiful harmony with instinct and habits; the teeth are continually growing from the base during the lifetime of the animal. The modifications of the molar teeth are not less remarkable; in all of them we find peculiar interblending of the cement, enamel, and dentine, so that a rough surface is secured by unequal abrasion. Still farther, their mode of implantation has direct relation to differences in the kind of food which the animal is led to seek and is able to digest. The rats, which subsist on mixed food, are less liable to the general wearing of the molars, "and no more dental matter is produced than is necessary for the firm implantation of the tooth in the jaw." Those Rodentia which feed on hard vegetable substances have molars which are liable to continual abrasion, but which are also continually growing at the base during most of the animal's life; in some, as in the Capybara, this renewal of the molars continues during the whole life. Professor Owen describes an additional

1 Owen's Odontography, p. 401.

arrangement in these permanently-growing molars, "they are curved, and the pressure during mastication is thus not directly transmitted to the formative pulp.”1

In carnivorous animals, certain teeth are admirably fitted for seizing, holding, and destroying the living prey, and others for dividing it afterwards. The incisors are of the typical number, and therefore more numerous than in many other mammalia, and they often present a transition in general form towards the canines. These latter are large, sharply conical, and of great strength. Some of the molar teeth present remarkable modifications. The fourth premolar in the upper, and the first molar in the lower jaw, are large and sharp-edged, so that when they are moved in opposition to each other, they cut like the blades of scissors; their function is sectorial, and they are admirably fitted to divide flesh. The concurrence of independent circumstances is seen in this, that the instincts of these animals lead them to eat flesh, the teeth are exactly suited to such food, their stomach is able to digest it, and the structure of their limbs enables them to seize and hold it. The hyæna obtains its food from the harder parts left by other carnivora, the bones of animals forming its chief sustenance; and the teeth are modified in harmony with its habits, presenting a remarkable deviation from the usual typical form in the carnivorous division to which the animal belongs. Certain of the premolars in upper and lower jaws are large and conical, and have at the base, near the line of the gum, a thick belt or ridge, which serves to protect that part during the process of crushing the hard bones. Professor Owen states that such a tooth, when shewn to an experienced engineer, was declared by him to be a perfect model of a hammer for stone-breaking.

1 Owen's Odontography, p. 402.