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.
Fig. 286. Nautilus
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 |