‖n. pl. [ NL. See Thorax, and Ostracoid, a. ] (Zool.) An extensive division of Crustacea, having a dorsal shield or carapec&unr_; &unr_;&unr_;niting all, or nearly all, of the thoracic somites to the head. It includes the crabs, lobsters, shrimps, and similar species. [ 1913 Webster ]