Spinacanthus cuneiformis is an extinct prehistoric tetraodontid bony fish that lived from the Lutetian epoch of Eocene Monte Bolca.

[1] In life, it would have resembled a somewhat-flattened boxfish with five long spines along the anterior-dorsal side, with the longest spine directly above the forehead, and the shortest spine directly in front of the dorsal fin.

(In Protobalistum, the scales are large, and form a sort of armor).

S. cuneiformis and Protobalistum were a part of the ecosystem of the lagoon that became Monte Bolca.

Because of their similarity to boxfish, and due to their close relation to modern-day triggerfish, spinacanthids may have preyed on shellfish and small fish.