fornix
From ZooTerms (Dictionary of Invertebrate Zoology)
fornix (noun; plural fornices; Latin fornix, vault): 1. An arch or fold.
2. (Arthropoda: Crustacea) A ridge in the lateral part of the cephalon above insertion of antennal muscles in water fleas.
3. (Mollusca: Bivalvia) The cavity on the inside under the hinge; the upper or convex shell of an oyster.
