a. Zool. [f. Gr. τρόπις, τροπιδ- keel + -IAL.] Pertaining to the tropis or keel of a C-shaped sponge-spicule.
1887. Sollas, in Encycl. Brit., XXII. 418/1. (Sponges) The pterocymba is subject to considerable modifications;… the pteres may be lamellar or ungual; additional lamellæ (tropidial pteres) may be produced by a lateral outgrowth of the keel.
