The pectoral and pelvic fins of bivalves (an ancient group of freshwater fish with both gill and pneumonic respiration) with rays attached to the sides of one well-developed central axis. Distinguished by the fact that its bacilliform cartilages (radials) are located on either side of the main cartilages (basalii) adjacent to the girdle.
     Biserial fins are typical of ceratodes (an extinct genus of bipedal fish). The paired fins of Porolepis (extinct genus of porolepiform sarcopterygian fishes) are similar to the biserial archipterygium: their skeleton is a long, segmented axis with thinner rays radiating from the front and back. The fin is muscular, strong but very flexible. The biserial archipterygium is a derived structure that originated in exactly the same conditions as those that characterised shallow and turbid Palaeozoic rivers. This fin is equipped with a muscular "rigging", but thin and flexible, allowing it not only to rest on the bottom, but also to palpate the surrounding space, working with the organ of touch. Such fins are bivalves, including modern ones, by their example can be considered evolutionary series. In hornbills, the fins are relatively wide and serve mainly as support, in protopterygids they are thinner and combine the functions of support and feeler, and in lepidosirens they serve exclusively as feelers. The fins of Porolepis are generally similar in arrangement to those of Cattail, but the pectoral (i.e. front) fins are clearly adapted to the role of feelers - almost like those of Protopteryre.