Source : Webster's Revised Unabridged Dictionary (1913)

Thoracostraca \Tho`ra*cos"tra*ca\, n. pl. [NL. See {Thorax}, and
   {Ostracoid}, a.] (Zo["o]l.)
   An extensive division of Crustacea, having a dorsal shield or
   carapec? ??niting all, or nearly all, of the thoracic somites
   to the head. It includes the crabs, lobsters, shrimps, and
   similar species.