Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
subtĕrĭor, us, comp. [subter], that is farther beneath, lower, = inferior (late Lat.): termini, Innoc. Cas. Lit. p. 232 Goes.: per subteriora, Aem. Mac. 2, 13.
Hence, adv.: subtĕrĭus, lower, farther down: subterius nigro, superius mineo, Isid. Orig. 16, 8, 4.