Cavolinia tridentata (Niebuhr, 1775) forma australis (Péron, 1807)
Shell hyaline or opaque, with flat dorsal side and moderately developed ribs. Ventral side moderately vaulted, with sculpture of faint growth lines and very faint transverse striation. Shell margin between caudal and lateral spines irregularly concave. Strong, whitish keels along lateral sides. Tips of lateral spines directed caudally. Caudal spine straight. Dorsal aperture lip separated from dorsal side by prominent arch-shaped rib between closing mechanisms, strongly truncated, usually ending in two anterior projections. Shell aperture trapezoidal, with brown to dark brown or purple pattern over the ventral and dorsal sides; dorsal aperture lip always lighter in color. Shell length: 16 mm, width: 10.5 mm.