Among all the mussels of Missouri, this is perhaps the easiest to recognize: As the shell grows, large knobs are produced, first on one shell and then on the other, in an alternating pattern.
Small, moderately thick and rounded shell with 3 to 4 large knobs or horns on each valve (shell half), alternating with those on the opposite valve; distinct depressions above each knob. Umbo is raised above hinge line. Epidermis is light tan to green, darker with age; often with fine green rays. Inside shell beak cavity is shallow to moderately deep; pseudocardinal teeth heavy, serrate and well developed; lateral teeth grooved, relatively long and straight to slightly curved; nacre (lining) white.
Similar species: Pimpleback and wartyback are similar, but neither has the fine green rays of the threehorn wartyback, nor such large, distinct knobs with associated depressions.