Shell. Medium to low profile; apex subcentral. All slopes convex. Sculpture of fine radial riblets may be eroded on larger specimens (20 mm). Aperture slightly narrowed anteriorly. Color gray to brown with oval white markings, sometimes drawn out into bars or rays at margins. Interior margin brown with white markings that correspond to exterior markings; intermediate area white; interior stain brown, irregular. Length, 10-45 mm.
Animal. Brown shading on tentacles.
Radula. First lateral teeth closely set on anterior edge of ribbon segment; elongate, pointed distally. Second lateral teeth posterior to first lateral teeth; elongate, pointed distally. Third lateral teeth lateral to second lateral teeth; wedge-shaped, reduced. Lacks marginal teeth. First lateral plates ovoid; second lateral plates irregular with convex medial edge, separated from third lateral plates by complete suture. Third lateral plates lateral to second lateral plates; posterior edge lobate. Ventral plates irregular, lateral edge concave, anterior process elongate, anterior suture present.

Small specimens (<10 mm) of L. scutum have extremely fine riblets, large oval white markings, and strongly anterior apices. These specimens occur in lower intertidal areas. In the field, specimens of L. scutum can be identified by the brown shading of the cephalic tentacles, a character that is not found in other central California acmaeids. This is especially useful with heavily encrusted specimens. On the outer coast small specimens of L. scutum (<15 mm) are sometimes confused with tessellate forms of Lottia patina. The tessellate form of L. patina is not known to occur in San Francisco Bay, and the coralline form of L. patina is readily distinguished from specimens of L. scutum by tentacular coloration. Finally, the radular configuration of this species is readily distinguishable from other species. The only species with similar radular configuration is L. persona, which has blunt rather than pointed first and second lateral teeth.

Akkeshi Bay, Hokkaido, Japan (43°N,145°E), Kurile Islands, Aleutian Islands, Alaska, to San Pedro, California (33° 307^) (McLean, 1966:94). San Francisco Bay: San Francisco County—St. Francis Yacht Club Breakwater, Aquatic Park, Fort Point, Alcatraz Island, Angel Island; Marin County— Sausalito Breakwater, Tiburon Point, Peninsula Point, Cone Rock; Contra Costa County—Castro Rocks.