Mesacanthus ('middle spine') is an extinct genus of acanthodian fish from early Devonian Scotland. It is among the more primitive of the Devonian acanthodians. Mesacanthus have pectoral, pelvic, anal and dorsal fin spines, as well as a pair of prepelvics (intermediate spines between the pectoral and pelvic fin spines) and small, unornamented, diamond shaped scales. According to Agassiz, the genus also has a distinct tail in which the upper lobe extends to a sharp point and the lower lobe forms a small triangle. Overall the genus is small (average length = 30mm) and fairly conservative for acanthodians.