Shell microstructure of other species Pleurolucina hendersoni, P. undata, Lucina pensylvanica and Cardiolucina quadrata. A Pleurolucina hendersoni Guadeloupe, fractured section with prominent calcified conchiolin layer, periostracum at base. Scale bar = 20 µm B P. hendersoni, detail of conchiolin layer with lines of calcareous spherulites. Scale bar = 20 µm C Pleurolucina undata Baja California, fractured section with thin conchiolin layer Scale bar = 200 µm D P. undata, detail of conchiolin layer with spherulites. Scale bar = 20 µm E P. undata, single spherulites embedded in conchiolin. Scale bar = 3 µm F Lucina pensylvanica Florida Keys, calcified conchiolin layer. Scale bar = 20 µm G L. pensylvanica, single spherulite. Scale bar = 2 µm H L. pensylvanica, section of periostracum with calcareous granules. Shell interior to top. Scale bar = 20 µm I Cardiolucina quadrata Philippines, fractured section with conchiolin layer. Scale bar = 200 µm J C. quadrata detail of conchiolin layer with calcareous aggregates. Scale bar = 50 µm K C. quadrata detail of calcareous aggregate. Scale bar = 10 µm.