Apex (mollusc)

The apex is a word most often used to mean the tip of the spire of the shell of a gastropod. It is the first-formed, and therefore the oldest, part of the shell. To be more precise, the apex would usually be where the tip of the embryonic shell or protoconch is situated, if that is still present in the adult shell.

The word apex can also be used to refer to a similar part in other molluscan shells, however, the apex of bivalves is more usually known as the umbos, beaks or prodissoconch.

The apex of tusk shells is the small, open posterior end, and the opening itself is usually called the apical aperture.

The phrase apical whorls, or protoconch, means the whorls of the embryonic shell at the apex of the shell, especially when this is clearly distinguishable from the later whorls of the shell, otherwise known as the teleoconch.

The space under the apex of a patellate gastropod is called the apical cavity.