a: Plots of principal component analysis (PCA) of all analysed specimens. The top left graph shows the proportion of variance of each component. Figures on the background indicate reconstructed shell morphology, based on principal components. b: Plots of PCA of Iwakawa's specimens and old literature specimens. The morphospace occupation by specimens from Hirano et al. (2019b) is shown as coloured polygons. The top left graph shows the proportion of variance of each component. Background outlines indicate the reconstructed shell morphology, based on principal components.