The examples of excessive number of teeth are presented, in the order Bruta, by the Priodont Armadillo, which has ninety-eight teeth ; and, in the cetaceous order, by the Cachalot, which has upwards of sixty teeth, though most of them are confined to the lower jaw ; by the common porpoise, which has between eighty and ninety teeth ; by the Gangetic dolphin, which has one hundred and twenty teeth ; and by the true dolphins (Delphinus), which have from one hundred to one hundred and ninety teeth, yielding the maximum number in the class Mammalia.
Form.—Where the teeth are in excessive number, as in the species above cited, they are small, equal, or subequal, and of a simple conical form ; pointed, and slightly recurved in the common dolphin ; with a broad and flattened base in the gangetic dolphin (Inia); with the crown compressed, and broadest in the porpoise ; compressed but truncate, and equal with the fang, in the Priodon. The compressed triangular teeth become coarsely notched or dentated, at the hinder part of the series, in the great extinct cetaceous Zeu glodon. The simple dentition of the smaller Armadillos, of the Orycterope, and of the three-toed Sloth, presents a difference in the size, but little variety in the shape of the teeth, which are subcylindrical, with broad triturating surfaces ; in the two-toed Sloth, the two anterior teeth of the upper jaw are longer and larger than the rest, and adapted for piercing and tearing.
In almost all the other Mammalia, particular teeth have special forms for special uses : thus, the front teeth, from being commonly adapted to effect the first coarse division of the food, have been called cutters or incisors ; and the back teeth, which complete its com minution, grinders or molars ; large conical teeth, situated behind the incisors, and adapted by being nearer the insertion of the biting muscles, to act with greater force, are called holders, tearers, laniaries, or more commonly canine teeth, from being well developed in the dog and other Carnivora, although they are given, likewise, to many vegetable feeders for defence or combat : e.g. Musk-deer (fig. 580, VII.). Molar teeth, which are adapted for mastication, have either tuberculate, or ridged, or flat, summits ; and usually are either sur rounded by a fence of enamel, or are tra versed by enamel plates arranged in various patterns. Certain molars in the Dugong,
the Mylodon, and the Zeuglodon, are so deeply indented laterally by opposite longi tudinal grooves, as to appear, when abraded, to be composed of two cylindrical teeth ce mented together, and the transverse section of the crown is bibbed. The teeth of the Glyptodon were fluted by two analogous grooves on each side. The large molars of the Capybara and Elephant have the crown cleft into a numerous series of compressed transverse plates, cemented together side by side.
The teeth of the Mammalia have usually so much more definite and complex a form than those of fishes and reptiles, that three parts are recognised in them : viz. the "fang," the " neck," and the "crown." The fang or root (radix) is the inserted part ; the crown (corona) the exposed part ; and the construc tion which divides these is called the neck (cervix). The term " fang " is properly given only to the implanted part of a tooth of re stricted growth, which fang gradually tapers to its extremity ; those teeth which grow un interruptedly have not their exposed part se parated by a neck from their implanted part, and this generally maintains to its extremity the same shape and size as the exposed crown.
It is peculiar to the class Mammalia to have teeth implanted in sockets by two or more fangs ; but this can only happen to teeth of limited growth, and generally characterises the molars and premolars ; perpetually grow ing teeth require the base to be kept simple and widely excavated for the persistent pulp. In no mammiferous animal does anchylosis of the tooth with the jaw constitute a normal mode of attachment. Each tooth has its particular socket, to which it firmly adheres by the close co-adaptation of their opposed surfaces, and by the firm adhesion of the alveolar periosteum to the organised cement which invests the fang or fangs of the tooth ; but in some of the Cetacea, at the posterior part of the dental series, the sockets are wide and shallow, and the teeth adhere more strongly to the gum than to the periosteum ; in the Cachalot I have seen all the teeth brought away with the ligamentous gum, when it has been stript from the sockets of the lower jaw.