The Collaborative International Dictionary of English v.0.48:

Pectostraca \Pec*tos"tra*ca\ (p[e^]k*t[o^]s"tr[.a]*k[.a]), prop.
   n. pl. [NL., fr. Gr. phkto`s fixed + 'o`strakon shell of a
   testacean.] (Zool.)
   A degenerate order of Crustacea, including the Rhizocephala
   and Cirripedia.
   [1913 Webster]