The former are extinct, and are not represented in the modern ocean; the latter, although greatly reduced in variety and numbers, are still found in various parts of the sea. During all this period, no backboned animals existed; then there were no fishes or amphibians, no reptiles or birds, and no mammals; nothing but invertebrates. The Cambrian fauna shows steady progress, being decidedly more advanced in the upper divisions than in the lower ones. =Lower Silurian (Ordovician) Period.= The life characteristics of this period are very similar to those of the Cambrian. Brachiopods have developed extensively, though they have not yet reached their height of development. The Trilobites attained their greatest development in this period; but after the Lower Silurian these creatures commenced to decline in variety and numbers, and finally underwent extinction near the close of the Palæozoic era. In America no plants have been discovered above the grade of seaweeds, but a few of the higher Cryptogams are doubtfully reported in Europe. The character of the flora in a later (Devonian) period makes it highly probable that land plants were well advanced in the Lower Silurian period. Professor Scott states that the remains of land plants may be discovered at any time, though this must remain a matter of chance, inasmuch as all known Ordovician rocks are marine, and therefore not a favorable circumstance for the preservation of land plants. Foraminifera and Radiolaria were abundant in the seas of the Lower Silurian period. Sponges and Brachiopods are numerous and varied. Among Cœlenterates the Graptolites are very numerous and varied; the few and doubtful Cambrian Corals are succeeded by a considerable number of Ordovician genera and species. They were characteristically different from the reef-builders of modern seas. The Echinodermata have greatly increased in importance, being more numerous and varied. The _Cystids_ reach their maximum development in this period. The Crinoids greatly increase in numbers and variety. The Star-fishes greatly expand; and a new and higher order of Echinoderms, the _Sea-Urchins_, make their first appearance in the later Ordovician period, but under very primitive forms. Among Arthropods the Trilobites increase greatly in numbers and variety, and attain their maximum development in this period. The occurrence of a Centipede in Ordovician rocks indicates the interesting fact that terrestrial animal life had already begun.