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.

The argonaut; – also called paper nautilus. See
Argonauta, and Paper nautilus, under Paper.

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