Zool. Pl. tropides. [mod.L., a. Gr. τρόπις keel.] The ‘keel’ or middle part of a cymba or C-shaped sponge-spicule, between the proræ or ‘prows.’
1887. Sollas, in Encycl. Brit., XXII. 417/2. (Sponges) A truly C-shaped spicule…. The back of the ‘C’ is the keel or tropis; the points are the prows or proræ.
