It may not be out of place here to mention that rocks are usually placed in two great classes, those which have been subject to great heat, melted, or partly so, at one time, then cooled and hardened are called metamorphic or igneous rocks. To these belong such rocks as our granites. Those which have not been changed by heat are called sedimentary rocks, such as sandstones, limestones, etc. In the former class we find no fossils. If fossils ever existed there, the fusing of the rocks has destroyed them. Sedimentary rocks contain many fossils. The Archaean area contains no sedimentary rocks, hence no fossils. Between the close of the Archaean and beginning of the Silurian is a long interval of which we know nothing. If any rocks were formed during this interval they are in no place exposed to the surface of the earth as are portions of all other formations. Life evidently existed then, for at the close of this interval or rather at the beginning of the Silurian we find a large number of Invertebrates. There were corals, crinoids, brachiopods, lamellibranches, gasteropods, cephalopods, worms and crustaceans. All of these animals flourished during the Silurian. It was during the latter part of the Silurian that fishes first made their appearance. If they lived earlier than this they were of low organization and possessed no hard parts, and when they died they would entirely decay, leaving nothing to be preserved as fossils. Of course, no one lived then to give fishes easy common names, and so we only know them by the long, hard scientific names given by scientific men. These we will use as little as possible in this article. In classifying fishes they fall into a few large groups, as follows: The lowest fish in point of structure is the lancelet, a small, semi-transparent animal, with no hard parts, as teeth, spines or bones. We would not expect it to be preserved as a fossil and so we find none. The next group contains our lampreys and hag fishes. These are parasites. They vary in length from a few inches to more than three feet. With a mouth nearly circular they attach themselves to other fishes and feast upon their blood. The hag fish eats its way into the fish and remains there until its host is a living hulk of skin and bones. Fishes known as Pteraspids, thought by some scientists to belong to this group, are found in the upper part of the Silurian. The lampreys of the present day have no very hard teeth and their backbone is simply very soft cartilage. These ancient lampreys, called Pteraspids, had the head and part of the body covered with a coat of mail. Of these there flourished in the last days of the Silurian quite a number of species. 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