During all the many millions of years that constitute the Silurian periods, hosts of species lived and died; hosts of variations were induced in the living creatures by the ever-changing environment; so that at the close of this time the Silurian rocks show that some of the life-forms, lowly as they were, had yet climbed higher in the scale of organization. The later Silurian rocks reveal the remains of _an entirely new branch of living forms, a higher branch of animals_ (=Vertebrates=) than had ever before existed. These remains are those of =Ostracoderms= and =primitive Sharks=. But at that early time they held a very subordinate position among the hosts of living creatures; the Molluscan Orthoceratites were still the rulers of the seas; it was still the Age of Molluscs. =Devonian Period.= The next set of ages succeeding the Silurian time is known as the Devonian period. Many of the kinds of creatures living in the Silurian ages are also found in these ages, but under changed forms. The Devonian seas had a great abundance and variety of Corals and Crinoids. The Chain Corals have perished, but the Cup and Honeycomb Corals still live on with modified appearance. The Graptolites are almost extinct. The Orthoceratites still live on, much reduced in numbers and size. The Molluscan Cephalopods have been revolutionized. New forms have grown out of the variations of the past, so that now in Devonian times we have the introduction of the great Molluscan _Ammonite family_, under the forms known as _Goniatites_, which are characteristic of this and the succeeding Carboniferous ages. The Trilobites still continue under new forms, but much reduced in size and number. Brachiopods are abundant and diversified. In this age, for the first time, land plants become conspicuous. The Devonian forests consisted of the highest flowerless plants, such as ferns, horsetails, and club mosses; and also the lowest flowering plants, such as cycads, pines, cypresses, etc.—plants which have imperfect and inconspicuous flowers. In all ages, as now, land vegetation has been closely related with _insects_. Insects, though rare as fossils, are found in connection with the forests in the Devonian period. They are among the lower orders of the class, and are somewhat allied to cockroaches and dragon flies.