The ostracoderms comprise a small group of fishes about which very little is known. They appear to have been inhabitants of fresh-water streams as well as lagoons bordering the seas, and may have been related to the small sharks of the time. They lived during the Ordovician, Silurian, and Devonian periods, and left no descendants now recognized among living creatures. A much larger type of armored fishes is known as the arthrodires, a name which refers to a pointed neck and an arrangement of the armor plates to permit a movement of the head. These were the most ferocious fishes of the Silurian and Devonian seas, some of them reaching a length of twenty feet though most were much smaller. Their jaws were provided with formidable shearing and crushing plates instead of teeth. DEVONIAN PROGRESS The Devonian is one of the most outstanding of all periods from the viewpoint of life development. Dominance of the fishes is its greatest achievement, the invertebrates remaining about as they were and the higher vertebrates barely in evidence, but life on a large scale was no longer confined to the seas. Fresh-water fishes became prominent and land plants well established. The first forests appeared, with fern-like plants predominating although woody trees of several types and considerable size were included. It is quite possible that extensive land areas had been well supplied with vegetation during earlier times, but the delicate tissues of plants are far less likely to be preserved than the limy parts of animals. The fossil record, therefore, cannot be expected to reveal more than a suggestion of the progress made at this level of living. The story of plant life becomes much clearer in the next period when conditions were more favorable for the production and preservation of plant fossils. Land animals of the time are almost unknown. A few snails and scorpions have been found, and some footprints made by early amphibians. Insects probably were in existence although the evidence is not quite clear on this point. The increasing number of fresh-water fishes, however, may be regarded as a sure indication that inland conditions were becoming more favorable for plant and animal inhabitants of all kinds.