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All cephalopods are univalves and fossils can range in size from 1cm in length to
at least 10m in length. The presence of an external calcareous shell is the feature that is of most interest in the study of these animals. It tends to be divided into two main parts: the phragmocone and the chambers. The phragmocone is the more developed part of the shell, and is divided internally into chambers by septa. There are two chambers: the air chambers, that contain an gaseous mixture similar to atmospheric air but richer in nitrogen, and this supply is controlled by the animal to make the shell lighter or heavier, so that the vertical movements can be carried out. Also, the living chamber is the inhabited part of the shell, bounded by the last septum and open towards the outside in a way that varies according to the type of animal. The shells may be ornamented with concentric ribs and longitudinal lines, ribs or bumps; and all shell forms are found at most times during cephalopod evolution. |
| Figure 6: Some common types of cephalopod conchs.
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