heterostraci

Century Dictionary and Cyclopedia

1. An order of ostracodermous fishes. They have an exoskeleton of shagreen, plates or scales without bone-cells, each plate consisting of three layers, and bodies without paired appendages. They are all of Paleozoic age.

WordNet 3.0

1. n. extinct group of armored jawless fishes or fish-like vertebrate; taxonomy is not clear