In tracing the teeth upward from their simplest to their most complicated forms, we find a very few examples (solely among fishes—as, for example, the wrasse), in which teeth consist of a single tissue—a very hard kind of non-vascular dentine. Teeth consisting of dentine and vaso-dentiue are very common in fishes, the hard dentine being external, and performing the office of enamel. Dentine and cement, the latter forming a thick outer layer, constitute the grinding teeth of the dugong. hi the teeth of the sloth, the hard dentine is reduced to a thin layer, and the chie bulk of the tooth consists of vaso-dentine internally, and a thicl crust of cement externally. " The human teeth and those of tin carniverous mammals appear at first sight to be composed of den tine and enamel only; but their crowns are originally, and thei] fangs are always covered by a thin coat of cement. There h also commonly a small central tract of osteo-deutine in oh teeth. The teeth called compound or complex in mammalic differ as regards their composition from the preceding only by the different proportion and disposition of the constituent tis.
sues. Fig. 1 is a longitudinal section of the incisor of a d is the dentine, e the enamel, and e the cement, a layer of which is reflected into the deep central depression of the crown s indicates the colored mass of .tartar and particles of food which fills up the cavity, forming the `mark' of the horse-dealer."— Organic Nature, vol. i. p. 267. Far more complex forms of teeth than this may be produced by peculiar arrangements, chiefly inflec tions, of the tissues. Certain fishes, and a family of gigantic extinct batrachians, to which Owen has, from this remarkable peculiarity, given the name labyrinthodons (q.v.), exhibit this kind of complex ity in a remarkable degree. Another kind of complication is pro duced by an aggregation of many simple teeth into a single mass.
These compound teeth are most common in fishes, hut are occasion ally met with in mammals. The teeth of the Cape ant-eater ((nye teropus), depicted and described by Owen in Tice Circle of the Sciences, are of this kind, each tooth being composed of a congeries of long and slender prismatic denticles of dentine, which are cemented together. In the ele phant, the compound molars belong to this class, the denticles being in the form of plates vertical to the grinding surface, and transverse to the long diameter of the tooth. When the tooth is bisected vertically and longitudinally, the three substances, dentine, cement, and enamel, are seen blended together.
Our limited space forbids our entering into any details regarding the teeth of fishes, further than to remark that, in regard to their number, form, substance, structure, situation, or mode of attachment, they offer a greater and more striking series of vari eties than do those of any other class of animals. In all fishes, the teeth arc shed and and renewed, not once only, as in mammals, but frequently during the whole course of their lives; and, as prof. Owen observes, " this endless succession and decadence of the teeth, together with the vast numbers in which they often co-exist in the same fish, illustrate the law of vegetation or irrelative repetition, as it manifests itself on the first introduction of new organs in the animal kingdom." While comparatively few fishes are entirely devoid of teeth, we find that in the class of reptiles, the whole order of chelonia (tortoises and turtles), the family of toads (bufonida in the order of batrachia), and certain extinct genera of sauria (lizards) are toothless. Frogs have teeth in the upper, but not in the lower jaw. Newts and salamanders have teeth in both jaws and
upon the palate; and teeth are found on the palate as well as on the jaws of most ser pents. In most lizards, and in crocodiles, the teeth are confined to the jaws. The teeth in reptiles are for the most part simple, of a conical form, and adapted, as in the case of most fishes, for seizing and holding, but not for dividing or masticating the food. In no reptile does the base of the tooth branch into fangs; and, as a general rule, the base of the tooth is anchylosed to the bone which supports it. The completion of a tooth is soon followed by preparation for its removal and succession, the faculty of developing new tooth-germs being apparently unlimited in this class. For further details regarding the teeth of fishes and reptiles, the reader is referred to prof. Owen's invaluable Anatomy of the Vertebrates, 1866, vol. i. pp. 359-409. Birds having no teeth, we proceed to the consideration of the dental system of mammals—a class which includes a few genera and species that are devoid of teeth. The true ant-eaters (myrme cophaga), the pangolins or scaly ant-eaters (manis), and the spiny monotrematous ant eater (echidna), are strictly toothless. The ornithorhynchus has horny teeth, and the whales (balcena and balanoptera) have transitory teeth, succeeded in the upper jaw by whale-bone. The female narwhal exhibits nothing more than the germs of two teeth in the substance of the upper jaw; in the male, one of these germs becomes developed into the remarkable weapon which specially characterizes the animal, and to which its generic term monodon (single tooth), is due. In the great bottle-nose whale, in the adult state, there are only two teeth (here occurring in the lower jaw); whence the name hyperoodon bidens. The elephant has never more than one entire molar, or parts of two, in use on each side of the upper and lower jaws; to which are added two tusks, which are modified incisors, more or less developed, in the upper jaw. Some rodents have two grinders on each side of both jaws, which, added to the four cutting-teeth in front, make 12 in all; but the common number of teeth in this order is 20, although hares and rabbits have 28 each. The number of teeth, 32, which characterizes man, the apes of the old world, and the true ruminants, is the average one of the class mam malia; but according to prof. Owen, " the typical number is 44." "I have been led," he observes, "chiefly by the state of the dentition in most of the early forms of both carnivorous and herbivorous mammalia which flourished during the eocene tertiary periods, to regard three incisors, one canine, and seven succeeding teeth on each side of both jaws, as the type-formula of diphyodont* dentition."—On the Classification and Geographical Distribution of the Mammalia, 1859, p. 18. A few of the monophyodonts possess from 80 to 100 teeth. See the article 31AmArALI4. The hog, the mole, the gym nure, and the opossum, are among the few existing quadrupeds which retain the typical number and kinds of teeth. The formula expressing the number of the different kinds of teeth—viz., the incisors or cutting-teeth, the canines or dog-teeth, the premolars, and the molars or true grinder, commonly known as the dental formula, is described in the article DENTITION, in which the milk or deciduous teeth, and the order in which they appear, are also described. It is only in the mammals that we have a well-marked divi sion of the teeth into the four kinds of incisors, canines, premolars and molars, each of which claims a brief description.