the GNU version of the Collaborative International Dictionary of English
noun-plural. A division of gastropod Mollusca, including the chitons. The back is covered by eight shelly plates. Called also Polyplacophora. See Illust. under chiton, and isopleura.
The Century Dictionary and Cyclopedia
In Gegenbaur's system of classification, one of two primary divisions of the Mollusca, consisting of the chitons only.