a. rare–1. [Cf. UMBONAL a.] Of or belonging to the umbo of a shell.
1824. Q. Jrnl. Sci., XVII. 16. Four series of short oblique grooves, or of indented wrinkles, three of which are arranged in a direction with the umbonial slope.
So Umbonic a.
1877. Huxley, Anat. Inv. Anim., viii. 473. Sometimes the umbonic cone is prolonged and bent inwards.
