Spinacanthidae is an extinct prehistoric family of tetraodontid bony fish that lived from the Lutetian epoch of Eocene Monte Bolca. In life, either genus would have resembled a somewhat-flattened boxfish with five massive 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. Protobalistum is distinguished from its close, sympatric relative, Spinacanthus, in that its scales are large, and form a sort of armor. In Spinacanthus, the individual scales are relatively small, and do not touch each other.