Apex is an anatomical term for the tip of the mollusc shell

Mollusc shell

The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes...

 of a gastropod, scaphopod, or cephalopod

Cephalopod

A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

 mollusc.

Gastropods

The word "apex" is most often used to mean the tip of the spire

Spire (mollusc)

A spire is a descriptive term for part of the coiled shell of mollusks. The word is a convenient aid in describing shells, but it does not refer to a very precise part of shell anatomy: the spire consists of all of the whorls except for the body whorl...

 of the shell of a gastropod. The apex 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

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 shell or protoconch

Protoconch

A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...

 is situated, if that is still present in the adult shell (often it is lost or eroded away).

Coiled gastropod shells

The phrase apical whorls, or protoconch

Protoconch

A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...

, means the whorls that constitute 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.

Comparison of the apical part and the whole shell of Otukaia kiheiziebisu:

Limpet-like gastropod shells

The space under the apex of a patellate

Patellidae

Patellidae is a taxonomic family of sea snails or true limpets, marine gastropod molluscs in the clade Patellogastropoda.Patellidae is the only family in the superfamily Patelloidea...

 gastropod is called the apical cavity.

Scaphopods

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

Cephalopods

In orthocone

Orthocone

An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....

 cephalopods, the pointed end of the shell is called the apex, and shell growth is away from the apex and toward the aperture

Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

. The first chamber of the apex is sometimes called the protoconch

Protoconch

A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...

.

Bivalves

The apex of a valve of a bivalve is more usually known as the umbo

Umbo

Umbo may refer to:*A shield boss* umbo , at the top of some mushrooms*The Umbo of tympanic membrane - a part of the human body*Umbo is the part of a Bivalve shell which was formed when the animal was a juvenile...

 or beak. When the embryonic shell is still present that is known as the prodissoconch.