The structure of mammalian teeth varies greatly, from the simplest slender cones to enormous and highly complicated apparatus, and, in order to comprehend the significance of these differences, we must look a little more closely into the materials of which the teeth are constructed and the manner in which those materials are combined. In all primitive mammals and in many of the higher and more advanced ones (including Man) a tooth is composed of the _crown_, or portion which is exposed above the gum, and the _roots_, one or more in number, by means of which the tooth is firmly inserted in the jaw-bone; the roots are at least partly formed before the tooth comes into use. Such a tooth is said to be short or low-crowned, or _brachyodont_. In many plant-feeders, such as horses, cattle, elephants, beavers, etc., the teeth continue to grow in height for a long time and do not form roots until late in life, or perhaps not at all. Such teeth are said to be long- or high-crowned, or _hypsodont_, and in very many instances the development of brachyodont into hypsodont teeth may be followed through every step of the change. The advantage of the change is obvious in lengthening the animal’s life, especially in those which feed upon abrasive substances, like grass, for the growth of the teeth long continues to make up for the loss through wear. Serious trouble has often been caused for captive elephants by giving them too soft food, so that the growth of the teeth is not properly balanced by abrasion. Still another category of teeth is the _rootless_, which are of simple form, like those of an armadillo, and grow throughout life, never forming roots. The chisel-like, or _scalpriform_ incisors of the rodents do not cease to grow while the animal lives; they are kept of constant length by continual use, and the arrangement of harder and softer tissue is such that the sharp edge is maintained; through accident or malformation it sometimes happens that the upper and lower teeth fail to meet, then the continued growth causes them to form curved hoops in the mouth, locking the jaws and bringing death by starvation to the unfortunate animal. [Illustration: FIG. 46.—Dentition of Beaver (_Castor canadensis_). _m. 3_, last molar. _p. 4_, last premolar. _i._, scalpriform incisors; enamel face black, dentine in vertical lines.]