In the current study, we use the recent molecular phylogeny of the Pterioidea (Tëmkin 2010) for investigating the occurrence of flexible margins within the Pterioidea. Specifically, we seek to (1) investigate differences in the extent of prismatic flange and its organic content between taxa and also between different valves of the same individual to explore the occurrence of asymmetry; (2) explore the flexibility of prismatic material in a hydrated state; (3) consider the phylogenetic and possible adaptive significance of these traits.