Nautiloidea, the sub-order of Tetra-branchiata Cephalopoda (q.v.), of which Nautilus is the typical and only living member. It differs from the other sub-order Ammonoidea (q.v.) in that the sutures or lines that mark the division of the separate body chambers are usually simpler and less folded than in the latter group; the siphuncle, moreover, passes through the centre of the septa and is protected by a back growth from this, forming a collar. In addition to the living Nautilus, the order includes a large number of extinct forms, and dates from the Upper Cambrian period.