Varix (mollusc)
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Varix (Pl. varices) is a thickened axial ridge in the shell of some types of Littorinimorpha and Neogastropoda, located at intervals around the whorl, and formed by thickening of the outer lip at a resting stage in the growth of the shell. In most gastropods bearing varixes (e.g. Cassidae, this is essentially a thickening or swelling of the shell at that point. But in some genera, such as Chicoreus, Hexaplex, Pteropurpura and Pterynotus of the family Muricidae, and Biplex of the family Ranellidae, the varices are characterised by beautiful ruffles, frills or lamellae. Other genera, like Murex, are armed with protective spines, which are formed by the varices closing or curling around their axis.
Gastropods with varices are characterised by episodic growth - the shell grows in spurts, and during the resting phase the varix forms.
