delthyrium

from The Century Dictionary.

• noun In the Brachiopoda, the triangular pedicle-opening in the cardinal area of the ventral valve, extending from the beak to the hinge-line. It may remain open through life or become closed in early life by the growth of the deltidium or, in secondary stages when the deltidium is absent, by the development of the deltidial plates. See deltidium, *deltidial plates, and *deltarium.