CRUSTACEANS. Trilobites now reach their greatest development and more than eleven hundred species have been described from the rocks of this period. It is interesting to note that in many species the segments of the thorax have now come to be so shaped that they move freely on one another. Unlike their Cambrian ancestors, many of the Ordovician trilobites could roll themselves into balls at the approach of danger. It is in this attitude, taken at the approach of death, that trilobites are often found in the Ordovician and later rocks. The gigantic crustaceans called the EURYPTERIDS were also present in this period. The arthropods had now seized upon the land. Centipedes and insects of a low type, the earliest known land animals, have been discovered in strata of this system. BRYOZOANS. No fossils are more common in the limestones of the time than the small branching stems and lacelike mats of the bryozoans,--the skeletons of colonies of a minute animal allied in structure to the brachiopod. BRACHIOPODS. These multiplied greatly, and in places their shells formed thick beds of coquina. They still greatly surpassed the mollusks in numbers. CEPHALOPODS. Among the mollusks we must note the evolution of the cephalopods. The primitive straight Orthoceras has now become abundant. But in addition to this ancestral type there appears a succession of forms more and more curved and closely coiled, as illustrated in Figure 285. The nautilus, which began its course in this period, crawls on the bottom of our present seas. VERTEBRATES. The most important record of the Ordovician is that of the appearance of a new and higher type, with possibilities of development lying hidden in its structure that the mollusk and the insect could never hope to reach. Scales and plates of minute fishes found in the Ordovician rocks near Canon City, Colorado, show that the humblest of the vertebrates had already made its appearance. But it is probable that vertebrates had been on the earth for ages before this in lowly types, which, being destitute of hard parts, would leave no record. THE SILURIAN The narrowing of the seas and the emergence of the lands which characterized the closing epoch of the Ordovician in eastern North America continue into the succeeding period of the Silurian. New species appear and many old species now become extinct. THE APPALACHIAN REGION. Where the Silurian system is most fully developed, from New York southward along the Appalachian Mountains, it comprises four series: 4 Salina . . . shales, impure limestones, gypsum, salt 3 Niagara . . . chiefly limestones 2 Clinton . . . sandstones, shales, with some limestones 1 Medina . . . conglomerates, sandstones