Charles Darwin,
A Monograph on the Sub-class Cirripedia…
“ The Terga are remarkable for their variability in all the species; in many specimens of E. Kingii and modestus the basal margin on the carinal side of the spur is deeply hollowed out. The width and acumination of the spur varies in all the species. In E. plicatus and simplex this valve is remarkably like that of Tetraclita porosa. In some specimens of E. Kingii the terga and scuta are firmly calcified together.Structure of the Parietes and Radii.—As in Tetraclita, the two lateral compartments are necessarily broad. ”
