(Synonyms: Risella parvula Dunker, 1861; Trochus diminutivus Reeve, 1861.) Length, 5 mm; diameter, 5 mm. Shell: trochoid with a broad, flat base forming an angular periphery; spirally keeled; yellow-brown. Spire: whorls flat; suture indistinct. Sculpture: sharp spiral keels extending on to the base. Aperture: oval; umbilicus narrow. Color: variable — light brown to yellow, variously freckled with white.
These littorines are abundant in the supratidal region along all rocky shores with Nodilittorina picta and Littorina pintado, but they are more often found in pools and apparently extend farther seaward on benches than do the other species.
P. tantilla occurs throughout the Pacific and possibly also the Indian Ocean.