Bivalvia, Linnaeus, 1758

Webster, Nicole B & Vermeij, Geerat J, 2017, The varix: evolution, distribution, and phylogenetic clumping of a repeated gastropod innovation, Zoological Journal of the Linnean Society 180 (4), pp. 732-754 : 745

Bivalvia

Although we will not discuss the entire diversity of bivalve shell sculpture, a few groups of bivalves have sculpture more or less reminiscent of varices. In the Trigoniidae some species, such as Myophorella montanaensis (M. Jurassic, Callovian) , have sharp commarginal ribs on part of the shell which White (1880) (in Imlay, 1964) termed varices. These are clearly not varices, however, as they do not extend the breadth of the shell. The Spondylidae have spectacular spiny projections, but these cannot be seen as varices because they do not connect along the growth axis into a single unit and they appear more as separate spines along ribs. A few venerids such as Hysteroconcha possess strong commarginal ribs (and spines, but fewer of them), but lack intervarical elements or evidence of periodicity.The sculpture most reminiscent of varices is found in the coarse commarginal nodes of Swiftopecten swiftii (Bernardi, 1858) and a few fossil pectinids, which appear as periodic commarginal thickenings, mainly on the right valve; however, these are pleats in the shell rather than thickenings ( Hertlein & Grant, 1972) and so do not fit our definition of varices.