Alternately rising or subsiding, what was once the bottom of the ocean now forms the mountain crest, and whole islands and continents have been gradually worn away and whelmed beneath the waves of the sea, to arise and to be whelmed again. In every part of the world we are able to trace these repeated changes in the fossil remains imbedded in the strata that have been successively deposited in the sea, and then raised again above its level by volcanic agencies, and thus, by a wonderful transposition, the history of the primitive ocean is revealed to us by the tablets of the dry land. The indefatigable zeal of the geologists has discovered no less than thirty-nine distinct fossiliferous strata of different ages, and as many of these are again subdivided into successive layers, frequently of a thickness of several thousand feet, and each of them characterized by its peculiar organic remains, we may form some idea of the vast spaces of time required for their formation. The annals of the human race speak of the rise and downfall of nations and dynasties, and stamp a couple of thousand years with the mark of high antiquity; but each stratum or each leaf in the records of our globe has witnessed the birth and the extinction of numerous families, genera, and species of plants and animals, and shows us organic Nature as changeable in time as she appears to us in space. As, when we sail to the Southern Hemisphere, the stars of the northern firmament gradually sink below the horizon, until finally entirely new constellations blaze upon us from the nightly heavens; thus in the organic vestiges of the Palæozoic seas we find no form of life resembling those of the actual times, but every class “Seems to have undergone a change Into something new and strange.” Then spiral-armed Brachiopods were the chief representatives of the mollusks; then crinoid star-fishes paved the bottom of the ocean; then the fishes, covered with large, thick rhomboidal scales, were buckler-headed like the Cephalaspis, or furnished with wing-like appendages like the Pterichthys; and then the Trilobites, a crustacean tribe, thus named from its three-lobed skeleton, swarmed in the shallow littoral waters where the lesser sea-fry afforded them abundant food. From a comparison of their structures with recent analogies, it is supposed that these strange creatures swam in an inverted position close beneath the surface of the water, the belly upward, and that they made use of their power of rolling themselves into a ball as a defence against attacks from above. The remains of seventeen families of Trilobites, including forty-five genera and 477 species, some of the size of a pea, others two feet long, testify the once flourishing condition of these remarkable crustaceans, yet but few of their petrified remains, so numerous in the Silurian and Devonian strata, are found in the carboniferous or mountain limestone, and none whatever in formations of more recent date.