1. The embryonic shell, or first chamber, of ammonites and other cephalopods; such as, the octopus, squid, cuttlefish, or nautilus (a mollusk with numerous tentacles, a horny beak, and a spiral shell with gas-filled chambers for buoyancy), having a large head, large eyes, prehensile tentacles, and, in most species, an ink sac containing a dark fluid used for protection or defense.
2. The oldest part of a gastropod shell, representing the area where shell development started in a larva.