Agnatha, which means ”without jaws,” is a class of primitive fishes. These fish, the earliest of the vertebrates, not only had no jaws but also lacked paired fins. Neither did they have bony skeletons, but many of the numerous kinds were covered with thick, bony armor and scales. Often they are called ostracoderms which means ”bony skin.” The oldest fossils of ostracoderms come from Ordovician rocks of Wyoming and Colorado, but most occur in Silurian and Devonian rocks. One group ( cephalaspids) had bony head shields. They were probably bottom dwellers. They many members of another group (pteraspids) swarm open water. Anaspids, more fish-like types, were covered with bony scales. most ostracoderms measured only a few inches long. Some lived in fresh water, others in seas. Though not found in rocks younger than the Devonian, their two living representatives-Lampreys and hagfishes-suggest that later forms may have lacked hard parts, thus were not fossilized.