Abstract: This study examines the development of the delthyrium, pseudodeltidium and interarea, their growth during the early juvenile stages of ontogeny and the extrapolation of morphology from adult shells to the probable juvenile state. Examination of the development and shell microstructure of the cardinalia of early juvenile thecideide brachiopod ventral valves from Jurassic, Cretaceous and Holocene specimens suggests that the delthyrium develops early in ontogeny and that the initial development of the pseudodeltidium precedes that of the interarea. Also, until the interarea is formed, the postero-lateral flanks of the ventral umbo are palintropic. The development of the interarea can be seen to be a consequence of the lateral extension of the early juvenile hinge line. Initially, the pseudodeltidium consists solely of a thin plate of primary shell material. Comparison of the morphology of the pseudodeltidium of early juveniles with that of adults suggests that the initially curved lateral profile of the pseudodeltidium is retained, or even accentuated, in the ontogeny of lacazellines, but in all thecidellinines, with the exception of Pachymoorellina and Minutella, following the appearance of the interarea, the pseudodeltidium becomes flattened and often appears continuous with the interarea. However, we do not support any proposal that suggests that, in thecideides, only those forms in which the delthyrium is closed by a dorsally convex plate should be considered to possess a pseudodeltidium sensu stricto, mainly because of physiological differences and the prospect of possible taxonomic confusion in the future. Instead, we propose the term planodeltidium for a flat pseudodeltidium, typically developed in the thecidellinines, and rugideltidium for a convex pseudodeltidium, typically developed in the lacazellines, but also in Pachymoorellina and Minutella. Despite the presence of a rugideltidium, we believe the affinities of Minutella are more strongly with the thecidellinines and have included it in the new subfamily Minutellinae of the family Thecidellinidae.