It was pointed out in Chapter II ([p. 38]) that very often the teeth are all that remains to us of extinct genera and species of mammals, and it may be further noted that the teeth are very characteristic and often suffice to fix the systematic position of a genus. Since, therefore, the teeth play such an uncommonly important part as fossils and are so pre-eminently useful to the palæontologist, it is necessary to give some general account of them.
Among the mammals the teeth display a very great variety of size and form in accordance with the manner in which they are used. Primarily, the function of the teeth is to seize and masticate food, and the kind of food habitually eaten by any animal is well indicated by the form of its teeth. The beasts of prey have teeth adapted for shearing flesh and crushing bones; plant-feeders have teeth fitted for cropping plants and triturating vegetable tissues; insect-eaters have teeth with numerous sharp-pointed cusps, or it may be, no teeth at all, swallowing without mastication the insects which they capture, etc. Among animals that have similar diet there is very great difference in the form and elaborateness of the grinding apparatus and it is often possible to follow out the steps of evolutionary change, by which, from simple beginnings, a high degree of complexity has been attained. In addition to the uses of the teeth as organs of mastication, they frequently serve as weapons of offence or defence. In the flesh-eaters which capture living prey they are formidable offensive weapons, and the fangs of the Lion or the Wolf are instances familiar to every one; but the tusks of the elephants or the hippopotamuses have nothing to do with the taking of prey. Several Old World deer, which have no antlers or very small ones, possess scimitar-like upper tusks, with which they are able to defend themselves very effectually.
In the lower vertebrates, such as reptiles and fishes, the number of teeth is usually indefinite and they continue to be shed and replaced, as needed, throughout life; but in each species of mammal, aside from abnormalities, the number is fixed and constant. Mammalian teeth are very generally divisible into four categories: (1) the incisors, or front teeth, which in the upper jaw are inserted in the premaxillary bones, (2) the canines, or eye-teeth, which are never more than one on each side of each jaw, or four in all, (3) the premolars, called in Man the bicuspids, the anterior grinding teeth which have predecessors in the milk-series and (4) the molars, the posterior grinding teeth which have no such predecessors.
Fig. 44.—Dentition of Wolf, left side. i. 3, third incisor. C., canine. p. 1, first premolar. p. 4, fourth premolar. m. 1, first molar.
It is customary and convenient to express the numbers and kinds of teeth of a given mammalian species by means of a “dental formula”; for example, in Man the formula is: i 2/2, c 1/1, p 2/2, m 3/3, × 2 = 32; the reason for the multiplication by two is that the figures deal only with one side of the mouth and must be doubled to give the sum total. Just because, however, the two sides are alike, it is usual to take the doubling for granted. Written out in full, the formula means that Man has two incisors, one canine, two premolars and three molars on each side of each jaw, the horizontal line indicating the division between upper and lower teeth. The number of teeth is frequently not the same in the upper and lower jaws; for instance, the formula for the Sheep is: i 0/3, c 0/1, p 3/3, m 3/3, × 2 = 32; the total is the same as in Man, but the arrangement is entirely different. The meaning is that in the Sheep there are no upper incisors or canines, but three incisors and a canine are present in each half of the lower jaw, with three premolars and three molars on each side above and below. The Dog gives still another formula: i 3/3, c 1/1, p 4/4, m 2/3, × 2 = 42. What is called the typical formula for the higher terrestrial mammals above the grade of the marsupials and which is but rarely exceeded, is i 3/3, c 1/1, p 4/4, m 3/3, × 2 = 44, though most existing mammals have fewer teeth than this. Compared with the typical formula, the Dog has lost but two teeth, the third upper molar on each side, while Man and the Sheep have each lost twelve.
As every one knows from his own experience, mammals normally have two sets of teeth, the first, temporary, or milk-dentition, in the young animal, and the second, or permanent dentition, in the adult. The milk-dentition, when fully developed, consists of incisors, canines and premolars, which usually agree in number, though often not in form, with the permanent teeth which replace them in the adult. The milk-teeth are frequently more conservative than the permanent ones and retain ancestral characters which have disappeared in the adult series, thus affording welcome information as to lines of descent and steps of evolutionary change. While there can be little doubt that the development of more than one dentition, or set of teeth, is the primitive condition among mammals and was derived by inheritance from their lower vertebrate ancestors, in which there was an indefinite succession of teeth; yet there are many mammals in which the milk-dentition is greatly reduced or altogether lost. In some, the milk-teeth are shed and replaced before birth, in others only the germs of the milk-teeth are formed and never cut the gum, while in others again all traces of the temporary series have vanished. This complete loss of the milk-teeth, like the presence of a great number of simple and similar teeth in the dolphins and porpoises, or the total absence of teeth, as in the anteaters and whalebone whales, is a secondary and derivative condition, never a primitive one.
Fig. 44a.—First upper molar, right side of Deer (Odocoileus). On the left, the masticating surface; heavy black line, enamel. On the right, external side, showing crown and roots. Brachyodont.
