One or more internal varices are present in about half of all Micrarionta rufocincta shells examined, usually associated with the strong growth lines that represent interruptions of helical growth. Shell structure in M. rufocincta consists of an outer, radial crossed-lamellar layer; a middle, concentric crossed-lamellar layer; and an inner, radial crossed-lamellar layer. Deposition of the inner layer lags 1-2 mm behind deposition of the outer and middle layers. Most of the thickness of a varix results from an increase in the depth of the middle layer. Similar varices are found in the Californian Micrarionta beatula, M. feralis, Helminthoglypta sonoma, H. carpenteri, and H. petricola and the West Indian Hemitrochus varians and Polymita picta. In M. rufocincta, varix formation occurs shortly before the onset of dry season estivation. Its main function is probably to strengthen the lip of the juvenile shell and provide greater attachment area for mucous seals during times when the threat of desiccation is greatest.