a. [f. L. umbōn-, umbo UMBO + -AL.] Of, belonging to, situated near, the umbo; of the nature of an umbo.
1854. S. P. Woodward, Mollusca, II. 316. Shell smooth, oblong;… umbonal rib extending across the interior of the valve.
1866. R. Tate, Brit. Mollusks, ii. 12. In the interior of the shell, the following … are to be distinguished: The umbonal cavity corresponding to the umbo [etc.].
1888. Rolleston & Jackson, Anim. Life, 124. A line … drawn to the ventral margin from the centre of the umbonal region.
