The American Heritage® Dictionary of the English Language, 4th Edition
n. Any of several groups of extinct primitive jawless fishes of the Ordovician through the Devonian periods in North America and Europe, having bodies that were encased in an armor of bony plates.
the GNU version of the Collaborative International Dictionary of English
n. Any of several extinct fishlike jawless vertebrates having a heavily armored body; of the Paleozoic.
The Century Dictionary and Cyclopedia
Having a bony skin like a coat of mail; ostraciont, as a fish; pertaining to the Ostracodermi. Also ostracodermal, ostracodermous.
n. An ostraciont fish, as a member of the Ostracodermi; a plectognath of the suborder Ostracodermi.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. extinct fish-like jawless vertebrate having a heavily armored body; of the Paleozoic
Word Usage
"The oldest fossil related to all modern vertebrates, the ostracoderm a jawless, bony-plated fish, dates to about 490 million years ago."