Siphuncle
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The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnit
A shell of Nautilus pompilius in cross section. Septal necks are preserved, but the thin connecting rings have been degraded and lost.
These polished orthocerid nautiloid cephalopod fossils from Morocco have fully preserved septal necks and connecting rings outlining the shape of the siphuncle.
Orthocerida
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Orthocerida, from Ancient Greek ὀρθός, meaning "straight", and κέρας, meaning "horn", also known as the Michelinocerida, is an order of extinct orthoceratoid cephalopods that lived from the Early Ordo
