Intracrystalline proteins from the valves of Terebratulina retusa and Neocrania anomala were compared to examine the process of calcitic shell formation in articulated and inarticulated brachiopods respectively. Differences in amino acid composition between the two species provide clues to the molecular processes in the two systems. Protein profiles differ between T. retusa and N. anomala and may indicate which proteins are involved in the formation of different ultrastructures. The intracrystalline proteins are characterised in terms of possible calcium-binding activity and glycosylation as well as their influence of calcite formation in vitro.