However much the faunas of the various geologic periods may have differed from each other, or from the fauna which now exists, in their general aspect and character, they were all, if I may so speak, equally underlaid by the great leading ideas which still constitute the master types of animal life. And these leading ideas are four in number. _First_, there is the _star-like_ type of life--life embodied in a form that, as in the corals, the sea-anemones, the sea-urchins, and the star-fishes, radiates outward from a centre; _second_, there is the _articulated_ type of life--life embodied in a form composed, as in the worms, crustaceans, and insects, of a series of rings united by their edges, but more or less movable on each other; _third_, there is the bilateral or _molluscan_ type of life--life embodied in a form in which there is a duality of corresponding parts, ranged, as in the cuttle-fishes, the clams, and the snails, on the sides of a central axis or plane; and _fourth_, there is the _vertebrate_ type of life--life embodied in a form in which an internal skeleton is built up into two cavities placed the one over the other; the upper for the reception of the nervous centres, cerebral and spinal--the lower for the lodgment of the respiratory, circulatory, and digestive organs. Such have been the four central ideas of the faunas of every succeeding creation, except, perhaps, the earliest of all, that of the Lower Silurian System, in which, so far as is yet known, only three of the number existed--the radiated, articulated, and molluscan ideas or types. The fauna of the Silurian System bears in all its three great types the stamp of a fashion peculiarly antique, and which, save in a few of the mollusca, has long since become obsolete. Its radiate animals are chiefly corals, simple or compound, whose inhabitants may have somewhat resembled the sea-anemones; with zoophytes, akin mayhap to the sea-pens, though the relationship must have been a remote one; and numerous crinoids, or stone lilies, some of which consisted of but a sculptured calyx without petals, while others threw off a series of long flexible arms, that divided and subdivided like the branches of a tree, and were thickly fringed by hair-like fibres.