Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† nautĭlus or nautĭlŏs, i, m., = ναυτίλος, the nautilus, a shell-fish so called because it sails like a vessel, Plin. 9, 29, 47, § 88; cf. nauplius and naviger.