Ultrastructural studies of the epithelium associated with the delthyrium and its covers of living rhynchonellide and terebratulide brachiopods show that it is sharply differentiated into two types. The microvillous pedicle epithelium exudes a cuticle bounded by a triple-unit membrane and composed of mucopolysaccharide and chitin; the normally developed outer epithelium secretes periostracum and carbonate. At the junction between the two types which coincides with the delthyrial and notothyrial openings of the young shell, a proteinous membrane or layer is secreted and acts as a bonding sheet between the cuticle and shell. Expansion of the circumference of the junction occurs only at its intersections with a pair of fused mantle lobes responsible for the growth of the cardinal margins. As expansion takes place the junction migrates into the delthyrial cavity leaving behind realigned secondary fibres which are normally coated with a thin carbonate film as well as a cuticular fold; this zone constitutes the pedicle collar. The ventral outer mantle lobes also grow dorso-medianly from their intersection with the pedicle-outer epithelial junction and secrete a pair of tetrahedral structures, the deltidial plates. Each plate is bounded by an outer surface of primary shell, an inner surface of pedicle collar and an oblique anterior surface of secondary fibres. The deltidial plates can extend medianly only because they grow dorsally of the umbo of the brachial valve. Further growth leads to a median conjunction of deltidial plates and, in some species, to a median fusion of the paired ventral lobes to form a common secretory unit exuding both periostracum and carbonate shell as a continuous structure (symphytium) across the delthyrium. Transverse sections near the posterior margin of the symphytium will always show this origin by fusion of ventral mantle lobes.