Partially phosphatized connecting rings show well-preserved structural details in the actinoceratoid Adamsoceras holmi (Ormoceratidae) from the Lower Ordovician (Kundan) of Estonia. Each connecting ring is composed of an outer, thin, spherulitic-prismatic layer, and an inner, thick, calcareous, lamellar layer, the latter being traversed by numerous large pores. The calcareous lamellar inner layer occurs in the connecting rings of three other actinoceratoids: Eushan-tungoceras pseudoimbricatum, Rayonnoceras solidiforme, and Huroniella sp. In Huroniella sp. and R. solidiforme, this layer is traversed by similar large pores as in Adamsoceras holmi, but in E. pseudoimbricatum the pores were probably too narrow to be recognized. The described structural type of the connecting rings, together with well-developed annular endosiphuncular deposits, are here considered as characteristic for actinoceratoids. The inflated connecting ring in actinoceratoids had a great surface area, which increased the number of pores across it. The permeability and emptying rates of the cameral liquid through the connecting ring could therefore have been as high or higher than that in Nautilus. In being composed of numerous calcified lamellae, the mechanical strength of the connecting ring against hydrostatic pressure could have also been the same or higher than that in Nautilus.