The _fauna_ of the Carboniferous system is a very numerous one, exhibiting specimens of almost every class of animal life, from the tiny _foraminifer_ up to the massive bone-covered sauroidal fish, and even to occasional traces of true reptilian remains. By far the larger number are peculiar to the sea, such as the molluscan tribes and corals; others are undoubtedly terrestrial organisms, such as the wings and wing-sheaths of several kinds of insects; while some appear to be peculiar to fresh or brackish water, such as shells allied to our _unio_ or river-mussel, and minute crustaceous animals known as _cyprides_, of which we have still representatives in our ponds and ditches. It is plain, then, that if we rightly ascertain the class or family to which one of these fossils belonged, we shall obtain a clue to the history of the physical geography, during Carboniferous times, of the district in which the fossil occurs. A bed of unios will tell us of old rivers and lakes that spread out their blue waters where now, perchance, there lie waving fields of corn. A bed of corals and stone-lilies will lay before us the bottom of an ancient ocean that rolled its restless waves where to-day, perhaps, the quarryman plies his task amid the gloom of dark pine-woods. In short, these organic remains are to the history of the earth what ancient monuments are to the history of man. They enable us to trace out the varied changes of our planet and its inhabitants down to the human era, just as the wooden canoe, the flint arrow-head, the stone coffin, the bronze sword, the iron cuirass, the ruined abbey, and the feudal castle, teach us the successive stages of progress in the history of our own country.