The Placoid, or _Plagiostome_ fishes, are familiar to us all as exemplified in the common thornback and skate of our markets. They are covered with a tough skin, which either supports a set of tuberculed plates as in the thornback, or a thick crop of small rounded bony points or plates, as in the shagreen of the sharks. The head consists of a single cartilaginous box. The spinal column is likewise formed of cartilage, built up in the higher genera of partially ossified vertebræ. The tail is heterocercal or unequally lobed, inasmuch as the spinal column, instead of ending off abruptly as it does in the herring, trout, and all our commoner fishes, passes on to the extreme point of the upper half of the tail. This is a noticeable feature, for it has been found to characterize all the fishes that lived in the earlier geological periods. The fins are often strengthened by strong spines of bone, which stand up in front of them and serve the double purpose of organs of progression and weapons of defence. The teeth vary a good deal in form. In the larger number of existing placoids they are of a sharp cutting shape, often with saw-like edges. Among the sharks they run along the jaws in numerous rows, of which, however, only the outer one is used, those behind lying in reserve to fill up the successive gaps in the front rank. The teeth do not sink into the jaw, as in the ganoids, but are merely bound together by the tough integument which forms the lips. Another form of tooth, abundant among the ancient placoids, and visible on some of those at the present day, shows a smooth rounded surface, the teeth being closely grouped together into a sort of tessellated pavement which, in the recent species, runs round the inner part of the jaws, while a row of conical teeth guards the entrance of the mouth. [Illustration: Fig. 28.--Ctenacanthus hybodoides. (Edgerton.)] The animals which possess these characteristics include the various tribes of the sharks and rays, and form the highest group of fishes. They are all active and predaceous, frequenting every part of the ocean where their prey is to be found. The formidable spines and hideous "chasm of teeth" belonging to the bulkier forms, render them more than a match for any other denizens of the deep, and thus they reign in undisputed supremacy--the scourge of their congeners, and a terror to man.