Shell microstructure of Pleurolucina harperae. A Fractured section of shell margin showing major notch growth halt and conchiolin layer. Scale bar = 400 µm B Fractured section showing succession of shell layers. Shell exterior at top. Scale bar = 100 µm C Conchiolin layer with regular bands of spherulites. Scale bar = 40 µm D Individual spherulite. Scale bar = 2 µm E Adjacent spherulites embedded in conchiolin with narrow channels between layers. Scale bar = 5 µm F Single spherulites with channels below and above. Scale bar = 5 µm. cl crossed lamellar layer co conchiolin layer cp composite prismatic layer ip irregular prismatic layer p periostracum sp spherulitic prismatic layer.