Illustrations of Shell Features
In some shells spiral growth will pause several times, the lip will thicken and it may appear spiral growth has stopped. But, then spiral growth begins again. This creates what appears as major thickened axial ridges (may include complex ruffles, frills, spines or lamellae) on earlier whorls. An axial ridge created via this mechanism is called a varix (varices). The final varix is the thickened outer lip of the mature shell.
See also discussion in section "The Adult Whorl."