The Collaborative International Dictionary of English v.0.48:

Thoracostraca \Tho`ra*cos"tra*ca\, n. pl. [NL. See Thorax, and
   Ostracoid, a.] (Zool.)
   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.
   [1913 Webster]