The characteristic fossils of the Silurian system are entirely unique. The trilobite may fairly be regarded as the prominent one; besides which there are orthoceratites, and graptolites, some members of the crinoidean family, with different kinds of corallines, and some other names to be rendered familiar only by future further study. We shall confine ourselves to those that our own recent researches have made us familiar with. First, here is the trilobite. We need not perplex our readers by any of the numerous subdivisions of this remarkable animal’s nomenclature; that would defeat the purpose of this book. Any work on geology will do this.[29] Here are three trilobites: one (1) by itself; another, (2) imperfect in its bed or matrix, and a third (3) rolled up. [Illustration] This most remarkable crustacean possessed the power of rolling itself up like the wood-louse or the hedgehog; and, reasoning by analogy, we suppose this to have been its defence against its numerous enemies. It is a very abundant fossil, found all over Europe, in some parts of America, at the Cape of Good Hope, but never in more _recent strata_ than the Silurian. The hinder part of the body is covered with a crescent-like shield, composed of segments like the joints of a lobster’s tail; and two furrows divide it into three lobes, whence its name.[30] Most remarkable are the eyes of this animal, and it is the only specimen in the vestiges of ancient creations in which the eye, that most delicate organization, is preserved; and if, as we believe, this little creature was living and swimming about, now and then fighting with some greater Cephalopodous mollusk, millions and millions of years ago, then in this fact we have the real fossil poetry of science, the romance of an ancient world which geology reveals to our delighted and astonished minds. From Buckland’s Bridgewater Treatise we give a drawing of the eyes of the trilobite; and in Buckland’s words we add: “This point deserves peculiar consideration, as it affords the most ancient, and almost the only example yet found in the fossil world, of the preservation of parts so delicate as the visual organs of animals that ceased to live many thousands, and perhaps millions of years ago. We must regard these organs with feelings of no ordinary kind, when we recollect we have before us the identical instruments of vision through which the light of heaven was admitted to the sensorium of some of the first created inhabitants of our planet.”[31] [Illustration: Fossil]