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

The rocks of these series are shallow-water deposits and reach the total thickness of some five thousand feet.  Evidently they were laid over an area which was on the whole gradually subsiding, although with various gentle oscillations which are recorded in the different formations.  The coarse sands of the heavy Medina formations record a period of uplift of the oldland of Appalachia, when erosion went on rapidly and coarse waste in abundance was brought down from the hills by swift streams and spread by the waves in wide, sandy flats.  As the lands were worn lower the waste became finer, and during an epoch of transition—­the Clinton—­ there were deposited various formations of sandstones, shales, and limestones.  The Niagara limestones testify to a long epoch of repose, when low-lying lands sent little waste down to the sea.

The gypsum and salt deposits of the Salina show that toward the close of the Silurian period a slight oscillation brought the sea floor nearer to the surface, and at the north cut off extensive tracts from the interior sea.  In these wide lagoons, which now and then regained access to the open sea and obtained new supplies of salt water, beds of salt and gypsum were deposited as the briny waters became concentrated by evaporation under a desert climate.  Along with these beds there were also laid shales and impure limestones.