Nautilus, pen. cor. Plinius. A certaine shell sish, in forme like a boate: and swimmeth the beliy vpward.
Lewis and Short: Latin dictionary
nautĭlus or nautĭlŏs, i, m., = nauti/los, the nautilus, a shell-fish so called because it sails like a vessel, Plin. 9, 29, 47, 88; cf. nauplius and naviger.