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They differ from the nautili in having the margins of the septa very much lobed or plaited, and the siphuncle dorsal.

After Professor Owen. a a, Partitions; b b, chambers; b', the last-formed chamber, in which the animal lives; c c, the siphuncle; d, attaching muscle; e e, the hollow arms; f f, retractile tentacles; g, muscular disk, or foot; h, the eye; i, position of funnel.

Carboniferous. a, Shell, reduced. b, Section, showing siphuncle.

The others are empty, and are connected with the body of the animal only by a pipe, or siphuncle, with membranous walls and filled with fluid.

The dotted line shows the position of the siphuncle.