Belantsea (bull-ANT-see-uh) was a kind of cartilaginous fish from the Carboniferous period that was related to the Permian Janassa. Both of these fish are chondrichthyans called Petalodonts. I first read about Belantsea in Paleo Sharks by Timothy J. Bradley.
Belantsea had a large, lobe-shaped fin on its ventral side, right behind the mouth. I believe that this may have helped Belantsea steer. Even though Belantsea could steer and move very well, it probably could not move very fast because of its shape, which was not hydrodynamic.
Belantsea had large teeth probably used to crush and eat hard-shelled animals such as corals and sponges. Fossils of Belantsea have been found in Montana and reach 2 feet in length. Fossils of Belantsea are rare.
Paleo Sharks: Survival of the Strangest by Timothy J. Bradley