A varix is an anatomical feature of the shell

Gastropod shell

The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

 of certain sea snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 gastropod molluscs.

It is a thickened axial ridge in the shell of some families of gastropods. A varix is located at intervals around the whorl

Whorl (mollusc)

A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the...

, and is formed by considerable thickening of the outer lip at a resting stage in the growth of the shell. In other words, in gastropods whose shells have varices, the shells are characterised by episodic growth - the shell grows in spurts, and during the resting phase the varix forms.

Gastropods whose shells have varices are primarily families and species within the taxonomic groups Littorinimorpha

Littorinimorpha

Littorinimorpha is a large clade of gastropods within Hypsogastropoda consisting primarily of marine species, but also aquatic and terrestrial species as well....

 and Neogastropoda

Neogastropoda

Neogastropoda is an unranked taxonomic clade of sea snails, marine gastropod mollusks. For many years prior to 2005 Neogastropoda was an order.- Description :...

.

In many gastropods bearing varices, for example the Cassinae

Cassinae

Cassidae is a taxonomic family of medium-sized, large and sometimes very large sea snails commonly called helmet snails or bonnet snails. These are marine gastropod mollusks in the superfamily Tonnoidea and the clade Littorinimorpha....

, the varix is essentially merely a thickening and swelling of the shell at that point. But in some genera within the family Muricidae

Muricidae

Muricidae, common name murex snails or rock snails, is a large and varied taxonomic family of small to large predatory sea snails. With approximately 1,600 living species the Muricidae represent almost 10% of the Neogastropoda. Additionally, 1,200 fossil species have been recognised...

, such as Chicoreus

Chicoreus

Chicoreus is a genus of medium to large sized predatory sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, the murexes or rock snails.-Species:...

, Hexaplex

Hexaplex

Hexaplex is a genus of medium-sized to large sea snails, marine gastropod mollusks in the family Muricidae, the murex shells or rock snails. -Species:Species within the genus Hexaplex include : * Hexaplex angularis...

, Pteropurpura

Pteropurpura

Pteropurpura is a genus of sea snails, marine gastropod mollusks in the family Muricidae, the murex snails or rock snails.-Species:Species within the genus Pteropurpura include:* Pteropurpura leeana...

and Pterynotus

Pterynotus

Pterynotus is a genus of sea snails, marine gastropod mollusks in the family Muricidae, the murex snails or rock snails.-Species:Species within the genus Pterynotus include:* Pterynotus alatus...

, and also within the genus Biplex of the family Ranellidae

Ranellidae

Ranellidae, common name the triton shells or tritons, is a taxonomic family of small to very large predatory sea snails, marine gastropod mollusks in the clade Littorinimorpha.-Subfamilies:...

, the varices are characterised by elaborate ruffles, frills or lamellae.

Some other genera, for example Murex

Murex

Murex is a genus of medium to large sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly calle "murexes" or "rock snails"...

, are armed with protective spines which may be straight or curved, and which are formed by the varices closing or curling around their axis.