In discussing this subject it will be necessary to say something about the geological history of the earth. Each geological age had its own peculiar fauna, and to write about any part of it means that we must know something about the particular geological age in which the animals under consideration of the earths previous history.

The earth is supposed to be a small, condensed portion of the gaseous material which astronomers telI us at one time pervaded all space. The heat given off when the gas was condensing has been largely converted into mechanical energy which makes the earth revolve once in twenty-four hours and sends it flying through space. As soon as the earth decreased to about its present size and became cool enough for water to be condensed on its surface, it began to write its own history. Its entire surface may have at one time been covered to a uniform depth by water. If such was ever the case it did not remain so long. The interior of the earth was very hot and the crust cooled very irregularly and portions of it rose above the surface of the water. Since then there have been two antagonizing forces at work. The heat has caused the earths surface to become irregular and — the water has made a strong effort, which has been partially successful, to reduce all irregularities to the same level. We do not know how long these forces nearly balanced each other, but sooner or later dry land appeared in many places on the earths surface. This was for a long period of time washed by heavy rains while the shores for some distance seaward were worn away by action of the tidal waves. Much of the land area then sank below sea level, and became covered with sand, gravel and the like, The portion which remained above the level is called the Archaean. Later a general elevation of the land area brought above sea level much of this land and gravel, forming around the Archaean an increased land era, which we call Silurian. The time when the sand and gravel was deposited forming this land is known as the Silurian age. Following this came the Devonian age. After this in the following order came the following geological ages: Carboniferous, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary and then the present or Recent age, the one in which we now live. Each of these ages is characterized by the peculiar animals which then predominated, and these animals are known only from their remains imbedded in the rocks as fossils.

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, semitransparent 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.