Calliarthron is a genus containing two species of thalloid intertidal[1] alga. Specimens can reach around 30 cm in size. The thalli take a crustose form. The organisms lack secondary pit connections. Calliarthron reproduces by means of conceptacles; it produces tetraspores, dispores and carpospores. The genus has lignin and contains secondary cell walls, traits which are normally associated with the vascular plants.[1] It is similar to the genus Bossiella.[2]
Calliarthron is calcified, but also has uncalcified joints that allow it to flex in response to the waves to which it is subjected.[1] These joints start out calcified, and decalcify as they grow older.[1] After decalcifying they grow much longer, then fatten themselves up in the same way as xylem formation, resulting in secondary walls.[1]