Tristychius
 Tristychius is an extinct genus of shark from the Carboniferous period. Fossils have been found in Scotland.Tristychius was a small shark about 60 centimetres (2 ft) long. It had a well-developed upturned caudal fin similar to that of many modern sharks. Physically it may have resembled a modern dogfish. Tristychius also had spikes attached to the bases of its dorsal fins probably for protection against predators.