(phylum Mollusca, class Cephalopoda)
A subclass of cephalopods which possess a multi-chambered, external shell composed of calcium carbonate, which is siphunculate (see siphuncle) and may be coiled. The gill structure is tetrabranchiate. Simple sutures are produced by contact between the internal septa and the shell wall. The subclass includes the oldest cephalopods, first recorded from Late 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 are more than 300 extinct genera and one extant genus, Nautilus, which dates from the Oligocene.
Subjects: Earth Sciences and Geography — Zoology and Animal Sciences.