Covering Terminology used in the Description of Externally Shelled Cephalopods (Nautiloids and Ammonoids)

By Kevin Bylund

Note: Kevin welcomes discussion on this article in the Cephalopod Fossils forum.

The structure secreted by the mantle of cephalopods for protection or neutral buoyancy is called the Shell or Conch. The complete shell is basically a hollow cone with two major parts, the Body Chamber, or Living Chamber, and the Phragmocone. The opening on the large end is called the Aperture, and the Apex is at the tip of the small end. The shell or Test that forms the cone is called the Shell Wall.

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Figure 1. Two lateral views of the shell of Nautilus, external on the left and internal on the right.



Orientation

Ventral
is the side the hyponome or siphon was on, usually identified by the Hyponomic Sinus, an indentation in the shell to let the hyponome protrude. Dorsal is the opposite side. Adoral, Adapertural, and Forward, are towards the aperture, Adapical and Backwardare towards the apex. Anterior is adapertural and Posterior is adapical. Lateral is between ventral and dorsal. Longitudinal is in an anterior to posterior direction, and Transverse is in a dorsal to ventral direction.

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Figure 2. Drawings of an imaginary coiled cephalopod shell -- lateral view on left (Part of the shell is broken away to reveal the suture lines on the internal mold) and...
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