† pseˈttaceous,a.Obs.rare—1. [f. L. psētta (Pliny), a. Gr. ψῆττα a turbot or other flat fish + -aceous.] Belonging to the group of flat fishes. 1661Lovell Hist.Anim. & Min.Introd., The Psettaceous, or plain and spinose, have a spine that seemeth to be divided in the midd'st.