Actinomma antarcticum group?
Three concentric shells. Cortical shell composed of a very irregular, sometimes sponge-like network of thick anastomosing bars. Second shell large, irregularly shaped, spongy. First shell small, circular, rarely visible. Cortical shell diameter: 200-400 µm.
Nigrini (1967) described Actinomma medianum, which differs from Actinommna antarcticum in that it has a simply latticed cortical shell and a more delicate medullary meshwork.
Ref.: Riedel (1958), Nigrini (1967).