1) Class 2) Class placodermi 
Arthrodira †Brindabellaspida †Petalichthyida †Phyllolepida †Ptyctodontida †Rhenanida †Acanthothoraci †?Pseudopetalichthyida †The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian. The various groups of placoderms were diverse and abundant during the Devonian, bu...
• (n. pl.) 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.
Plac`o·der'mi noun plural [ New Latin , from Greek ..., ..., a tablet + ... 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 . 
[n] - extinct group of bony-plated fishes with primitive jaws
class Placodermi noun extinct group of bony-plated fishes with primitive jaws