The Late Carboniferous bransoniid conocardioidean Apotocardium lanterna (Branson, 1965) had an entirely aragonitic shell with a finely prismatic outer shell layer, a predominantly crossed lamellar to complex crossed lamellar middle shell layer, and an “inner” shell layer of finely textured porcelaneous and/or matted structure. This “inner” layer is probably homologous with the inner part of the middle shell layer and the inner layer sensu stricto of bivalved molluscs. Shell morphological and microstructural convergences between conocardioids and living heart cockles suggest that at least some conocardioids may have farmed algal endosymbionts in their posterior mantle margins. This symbiosis may have helped conocardioids compete with the biomechanically more efficient bivalves during the latter part of the Paleozoic.