From ZooTerms (Dictionary of Invertebrate Zoology)
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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.