Figure 4.

(a) Thickness of the shell wall of both species, measured at the same distance from the aperture. Electron micrographs of fractured cross-sections of the shells of K. editha (b,d,f) and K. gainesi (c,e,g). (b,c) The entire cross-section, (d,e) the cross-lamellar arrangement close to the exterior side and (f,g) the cross-lamellar arrangement on the interior side. The yellow lines indicate the lamellae criss-crossing at approximately 125° and approximately 113°. The red arrows indicate crack deflection as the lamellae orientation changes at the interface between layers. (h,i) Elemental composition in wt% throughout the thickness x of the shells, for K. editha (h) and K. gainesi (i). The composition was obtained using energy-dispersive X-ray analysis (EDX) (full spectra in electronic supplementary material, figure S3).