1. (n.) The only existing genus of tetrabranchiate cephalopods. About four species are found living in the tropical Pacific, but many other species are found fossil. The shell is spiral, symmetrical, and chambered, or divided into several cavities by simple curved partitions, which are traversed and connected together by a continuous and nearly central tube or siphuncle. See Tetrabranchiata.

2. (n.) The argonaut; -- also called paper nautilus. See Paper.

3. (n.) A variety of diving bell, the lateral as well as vertical motions of which are controlled, by the occupants.