(n)fish-like vertebrate with bony plates on head and upper body; dominant in seas and rivers during the Devonian; considered the earliest vertebrate with jaws
‖n. pl. [ NL., fr. Gr. pla`x, plako`s, a tablet + de`rma skin. ] (Paleon.) An extinct group of fishes, supposed to be ganoids. The body and head were covered with large bony plates. See Illust. under Pterichthys, and Coccosteus. [ 1913 Webster ]