delthyrium

From ZooTerms (Dictionary of Invertebrate Zoology)
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delthyrium (noun; plural delthyria; Greek 4th letter, delta; thyrion, door): (Brachiopoda) The central triangular notch in the ventral valve, open to the hinge line; facilitating the passage of the pedicle; usually closed off from the hinge plate by the deltidium.