A varix (Pl. varices) is an anatomical feature of the shell of certain sea snails, marine 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, 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 Cassinae, 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 elaborate 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.