Requieniidae Kutassy 1934

FamilyRequieniidae Kutassy, 1934 Remarks. Requieniidae are diagnosed as strongly inequivalve, with a trochospirally to helicospirally coiled left (attached) valve, mainly differing from Epidiceratidae in the smaller angle between the coiling axis and the commissural plane; the latter, as a result, intersects the earlier whorls in adult requieniids (Gourrat et al. 2003; Scott et al. 2010; Skelton 2013; Fig. 3 B). The much smaller right valve is poorly inflated or entirely flat. Scott et al. (2010) reported a thicker outer calcitic shell layer as an additional diagnostic character. However, in Jurassic representatives, the outer shell layer seems to be equally thin to that in Epidiceratidae, and Plesiodiceras also has a similarly reduced, operculum-like right valve as most Requieniidae, making the coiling geometry the only reliably diagnostic character.