Varix (mollusc)

A varix (Pl. "varices") is an anatomical feature of the shell of certain sea snails, marine gastropod mollusks.

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

In many gastropods bearing varices, for example the Cassidae, the varix is essentially merely a thickening and swelling of the shell at that point. But in some genera within the family Muricidae, such as "Chicoreus", "Hexaplex", "Pteropurpura" and "Pterynotus", and also within the genus "Biplex" of the family Ranellidae, the varices are characterised by beautiful ruffles, frills or lamellae.

Some other genera, for example "Murex", are armed with protective spines which may be straight or curved, and which are formed by the varices closing or curling around their axis.