The next group of fishes are the sharks, the most blood-thirsty of all the inhabitants of the sea. Sharks flourished to some extent in the upper part of the Silurian. The shark has no true bones and its covering consists of shagreen tentacles. It is provided with hard teeth and the dorsal fins of the ancient shark were provided each with a hard, stout spine. The teeth were large, flat and fit for crushing. We know these ancient sharks only by the spines, shagreen tentacles and the teeth. These, however, furnished abundant evidence that the sharks in the upper Silurian were numerous as to individuals and species. The Chimera, a fish much resembling the sharks, was also abundant in the upper Silurian. A group of fishes usually known as ganoids and which comprise the lung fishes of the Nile, of Australia, the garpikes of North America and the sturgeons, were very abundant during the closing days of the Silurian. The fishes of this group are especially well preserved as fossils, their covering consisted of bony plates or bucklers or of scales covered with a coat of enamel. Their outer covering was well suited to become fossilized, and so we know this group much better than we do any other found in the Silurian.

The next and last group of fishes is known as Teleosts, or bony fishes. To this group belong our typical fishes, such as black bass, sun fishes, suckers, cat fishes and the like. None of this group lived during the Silurian.

Following the Silurian came the Devonian, which is called the age of fishes. In no time in the worlds history have fishes been so large and so abundant as during this age. They outclassed in every respect all other animals. The same general types flourished as those which existed in the latter part of the Silurian. There were more species and more individuals and some grew to an enormous size. Fishes ruled the Devonian seas. The crustaceans, such as trilobites which greatly predominated during the Silurian age, diminished, greatly during the Devonian. In the struggle for existence they decreased in size and in numbers, and were obliged to seek safety in less favorable places. The Devonian fishes were largely sharks and ganoids, especially the latter.

These were covered, with hard, enamel-coated scales or bony plates. Some were short and heavy and entirely encased in a covering of large bony plates. They were evidently anything but pretty and their movements in the water must have been extremely awkward. Others were formed much after our own ideas of fishes. These bore much resemblance to our garpikes, the lung fish of the Nile and the lung fish of Australia, and the worthless dog fish of our own fresh waters. Anglers and fishermen all despise these fishes now, yet in Devonian times the fishes most nearly like them were evidently the most handsome and graceful of all fishes then living. It appears as if fishes in those days did not fight each other. They found abundant sea room and plenty of food in the form of invertebrates. Of course it is quite probable that many fish-like animals existed at this time, but possessing no hard parts and were not preserved as fossils; these could not become at all important for the sea was too full of large animals of all classes which were so well protected with a coat of mail and so hostile that those less favorably situated could not exist in any great numbers. At the close of the Devonian many changes took place. The rocks of this formation, which now form a portion of the earths surface, rose Put of the water, the land area thus considerably increased, the seasons, such as they were then, became more marked; many inland seas were formed. These changes were more or less gradual, but not so much so that the fishes living then could not suit themselves to the new conditions. Those fishes which had flourished for generations had become accustomed to easy living and certain fixed ways, could not adapt themselves to changing conditions, and so became extinct.

The Pteraspids, the earliest forms to appear; the Pterichthys, in fact, all forms which bear any resemblance to our present lampreys, or which may prove a close relative of the earlier ganoids, became extinct at close of the Devonian. The early Chimeras which flourished from close of Silurian also became extinct. Many ganoids became extinct, but other ganoids came into existence to take their places. The ganoids most nearly like our modern sturgeons increased during the last of the Devonian and retained their prominence to the close of the Carboniferous. The slow-moving, heavily plated ganoids passed away. They ruled during the Devonian age, but could not suit themselves to the new conditions at beginning of the Carboniferous.