Subclass of cephalopods which possess a multichambered, external shell composed of calcium carbonate, which is siphunculate (see siphuncle) and may be coiled. The animal lives in the last-formed chamber, the body chamber. The gill structure is tetrabranchiate (four-gilled). Simple sutures are produced by contact between the internal septa and the shell wall. The subclass includes the oldest cephalopods, first recorded from Upper Cambrian rocks. They diversified and became common throughout the Palaeozoic, but were greatly reduced at the end of this era. Further diversification occurred in the Mesozoic, but the group dwindled again in the Cenozoic. There is only one extant genus, Nautilus, which dates from the Oligocene. See also plectronoceras cambria.