Tristychius is an extinct genus of shark from the Carboniferous period (Visean). Fossils of T. arcuatus, the type and only species, including fin spines 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.