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
. [1913 Webster
" The head of the animal bears numerous simple tapered arms, or tentacles, arranged in groups, but not furnished with suckers. The siphon, unlike, that of ordinary cephalopods, is not a closed tube, and is not used as a locomotive organ, but merely serves to conduct water to and from the gill cavity, which contains two pairs of gills. The animal occupies only the outer chamber of the shell; the others are filled with gas. It creeps over the bottom of the sea, not coming to the surface to swim or sail, as was formerly imagined."