interradial [paleont] (in-ter-ra'-di-al). adj. Situated midway between the axes of adjacent rays of an echinoderm; e.g. an "interradial suture" representing a common line or division between adjacent radial plates of a blastoid. n. (a) A plate in the interradial area; e.g. a crinoid plate above a basal. (b) interray.