The majority of people, if asked to give a definition of the meaning of teeth, would reply that they are hard structures that grow in the jaw. But this is an idea that requires very considerable modification from a scientific point of view. In the first place, they are found in other places besides the jaws; and in the second place, they are by rights structures originally belonging to the skin. Both these important facts must be illustrated by reference to the Fishes, which exhibit the primitive types of teeth. In fishes, not only are teeth found on the jawbone, but sometimes also on other bones which border upon the cavity of the mouth; they are found on the palatine bone, or roof-plate of the mouth, and, still more strange, upon bones which belong to the "hyoid apparatus," or skeleton of the gills (see above). The latter may form a set of throat-teeth, which are used for grinders, while the jaw-teeth are used for biting. Among the Carps, the jaw-teeth are reduced, and the fish depends upon its throat-teeth only. In the Wrasses, one pair of the bones that bear throat-teeth (the inferior pharyngeal bones) are fused, so as to form a stronger apparatus: and from this circumstance, the group of Fishes to which they belong has been given the name of Pharyngognathi, fishes possessing throat-jaws. They have, however, biting teeth as well, in the true jaws. The grinding teeth are apparently used for consuming the food in a leisurely manner when once it has been taken into the mouth. A curious circumstance in connection with these "throat-jaws" is, that they produce musical sounds. Fishes have other means, however, of producing a voice--usually by means of the swimming-bladder and muscles in connection with it. Probably they are able, to some extent, to effect communication with each other in this way. It has already been stated that teeth, in their primitive form, are to be regarded as skin-structures. Certain fish, which are looked upon as ancestral types, have, dispersed throughout the skin, a number of bony plates, or granules (placoid scales), more or less formidable, and tipped with a hard enamel-like substance. Teeth are regarded as but a special form of these. But if they are skin-structures, how come they in the mouth and throat? Because the mouth and throat are lined by an ingrowth from the external skin; the origin and growth of this is seen in the embryo. In the Mammalia the teeth, though restricted in number, attain the greatest possible variety of form, so that the jaws of different but allied species may be distinguished by their teeth.