On the Supposed Evolution of Gryphaea in the Lias

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Abstract
A re-examination of Hettangian and Lower Sinemurian oysters has led to the rejection of Trueman's hypothesis on the evolution of Liostrea into Gryphaea. Gryphaea assemblages from two horizons in different parts of Britain have been subjected to bivariate and size frequency analysis. The only detectable difference is a slight size increase up the succession. No “gryphaeoid” trend is discernible in Liostrea up the Hettangian succession and the change to Gryphaea is in consequence abrupt. All the oysters of Trueman's series are referred to what are considered to be two different Linnaean species.