The Collaborative International Dictionary of English v.0.48:

Holostraca \Ho*los"tra*ca\, n. pl. [NL., fr. Gr. "o`los whole +
   ? shell of a testacean.] (Zool.)
   A division of phyllopod Crustacea, including those that are
   entirely covered by a bivalve shell.
   [1913 Webster]