Prefabrication of shell ornamentation in the bivalve Laternula

The minute, cone-like spikes which cover the valve surfaces of Laternula arc formed by the mantle in advance of the growing shell margin and are subsequently incorporated into the calcarcous shell. This 'prefabrication' process allows the formation of a prismatic spike ultrastructure distinct from other layers of the nacro-prismatic shell and provides ornamentation to the otherwise uncalcified ventral surface. A consideration of a number of morphologic variables, including the mechanism of shell construction, suggests that spikes function to increase the skin friction of the valve surfaces and thereby promote a stable life position. Variations on Laternula's shell construction mechanism arc found throughout the Pandoracea and possibly larger taxonomic groupings.