orbiculoidea

from The Century Dictionary.

• noun A genus of neotrematous brachiopods of the family Discinidæ, having circular chitinous shells, the pedicle passage being in the form of a slit which extends from the beak of the pedicle-valve forward, and closed behind, as growth advances, by a listrium. It is common in all rocks from the Lower Silurian to the Cretaceous.