We should naturally conclude, from ob serving the great diversity in the general form of fishes, that the structure of their skeleton must be equally various. They agree together, however, on the whole, in having a spine, which extends from the cranium to the tail-fin ; and in having the other fins, particularly those of the thorax and abdomen, articulated with peculiar bones, destined to that purpose. They have in general many more bones uncon nected with the rest of the skeleton, than the animals of the preceding classes.
The cranium in several cartilaginous fishes, (in the skate for instance) has a very simple structure, consisting chiefly of one large piece. In the bony fishes, on the contrary, its component parts are very numerous ; amounting to eighty in the head of the perch. Most of the latter have a more or less moveable un der jaw.
Great variety.in the structure of the teeth is observed in this class. Some ge nera, as the sturgeon, are toothless. Their jaws, which are distinct from the crani um, form a moveable part, capable of being thrust forwards from the mouth, and again retracted.
Those fishes which possess teeth dif fer very much in the form, number, and position of these organs. Some species of sparus (as the S. probato-cephalus) have
front teeth almost like those of man ; they are provided with fangs, which are con tained in alveoli. In many genera of fishes, the teeth are formed by processes of the jaw-bones covered with a crust of enamel. In most of the sharks, the mouth is fur nished with very numerous teeth, for the supply of such as may be lost. The white shark has more than two hundred, lying on each other in rows, almost like the leaves of an artichoke. Those only which form the front row have a perpendicular direction, and are completely uncovered. Those of the subsequent rows are, on the contrary, smaller, have their points turned backwards, and are covered with a kind of gum. These come through the covering substance, and pass forward when any teeth of the front row are lost. It will be understood from this description that the teeth in question cannot have any fangs.
The saw-fish only (squalus pristis) has teeth implanted in the bone on both sides of the sword-shapen organ, with which its head is armed.
In some fishes the palate, in others the bone of the tongue (as in the frog fish,) in others (as in several of the ray-kind,) the aperture of the mouth forms a continuous surface of tooth.