Succeeding these, in Carboniferous times, came a host of shark-like creatures known mainly from their teeth and spines, for their skeletons were of cartilage, and belonging to types that have mostly perished, giving place to others better adapted to the changed conditions wrought by time. Almost the only living relative of these early fishes is a little shark, known as the Port Jackson Shark, living in Australian waters. Like the old sharks, this one has a spine in front of his back fins, and, like them, he fortunately has a mouthful of diversely shaped teeth; fortunately, because through their aid we are enabled to form some idea of the manner in which some of the teeth found scattered through the rocks were arranged. For the teeth were not planted in sockets, as they are in higher animals, but simply rested on the jaws, from which they readily became detached when decomposition set in after death. To complicate matters, the teeth in different parts of the jaws were often so unlike one another that when found separately they would hardly be suspected of having belonged to the same animal. Besides teeth these fishes, for purposes of offence and defence, were usually armed with spines, sometimes of considerable size and strength, and often elaborately grooved and sculptured. As the soft parts perished the teeth and spines were left to be scattered by waves and currents, a tooth here, another there, and a spine somewhere else; so it has often happened that, being found separately, two or three quite different names have been given to one and the same animal. Now and then some specimen comes to light that escaped the thousand and one accidents to which such things were exposed, and that not only shows the teeth and spines but the faint imprint of the body and fins as well. And from such rare examples we learn just what teeth and spines go with one another, and sometimes find that one fish has received names enough for an entire school. These ancient sharks were not the large and powerful fishes that we have to-day--these came upon the scene later--but mostly fishes of small size, and, as indicated by their spines, fitted quite as much for defence as offence. Their rise was rapid, and in their turn they became the masters of the world, spreading in great numbers through the waters that covered the face of the earth; but their supremacy was of short duration, for they declined in numbers even during the Carboniferous Period, and later dwindled almost to extinction. And while sharks again increased, they never reached their former abundance, and the species that arose were swift, predatory forms, better fitted for the struggle for existence. _REFERENCES_