Siphuncular structure in the Jurassic belemnitidMegateuthis(Cephalopoda: Coleoidea)

Each septum of the phragmocone in the Jurassic belemnitid Magateuthis is composed of a thick, lamello-fibrillar, nacreous layer and a thin, chitinous layer, here termed the additional chitinous layer, which covers the aboral surface of the septum. The nacreous and additional chitinous layers continue from the septum into the septal neck. The inner surface of the septal neck is covered by a thick, tubular, conchiolin layer that originates on the septal surface around the siphuncle. The connecting ring is composed of three conchiolin layers with high elasticity: the additional chitinous layer that fuses with the tubular chitinous layer and continues into the connecting ring, and an outer chitinous layer that is a non-calcified continuation of the nacreous layer of the septal neck and is often destroyed by diagenesis. As seen in higher magnification, the tubular layer consists of tightly packed tubes with chitinous walls that are oriented at right angles to the long axis of the siphuncle. At about half of it...