In Ammonoids the siphuncle is always marginal, and usually external.
It is peculiar to the louse tribe (Pediculid), and it consists of the tubulet (Tubulus), and siphuncle (Siphunculus).
Shell coiled in one plane, aperture wide and simple, siphuncle central.
Shell slightly curved, aperture simple, siphuncle wide, septa approximated.
Shell straight or curved, fusiform, aperture simple, siphuncle contracted at septa.
The siphuncle in the Nautilus pierces the centres of the septa, and in fossil Nautiloids it is usually central or sub-central.