Shell. Medium profile; apex positioned in anterior third of shell. Anterior and lateral slopes straight, posterior slopes convex. Sculpture of concentric growth lines, occasional specimens with fine riblets as well. Aperture anteriorly narrowed. Color green to gray with white tessellations, drawn out into fine radial markings at the shell margin, or blue-gray with white triangular markings. Apex typically eroded to white. Interior margin white with darker markings that correspond to exterior markings; intermediate area blue-white; lacks central stain. Length, 5-20 mm.
Animal. Lacks pigmentation, white.
Radula. First lateral teeth closely set at anterior edge of ribbon segments, pointed distally. Second lateral teeth posterior to first lateral teeth, broad, pointed distally. Third lateral teeth lateral to second lateral teeth, reduced, pointed distally, medial edges convex, lateral edges concave. Uncini large. First lateral plates subrectangular; second lateral plates irregular, posterior edge rounded, separated from third lateral plates by partial suture. Third lateral plates rounded. Ventral plates subrectangular. [Lindberg]

DISTRIBUTION
Southwestern Alaska to Northern Baja California, Mexico (23° N). San Francisco Bay: San Francisco County—Fort Point, St. Francis Yacht Club Breakwater, Black Point, Aquatic Park, Alcatraz Island, Angel Island, Yerba Buena Island; Marin County—Fort Baker, Yellow Bluff, Cone Rock, Peninsula Point; Contra Costa County—Castro Rocks; Alameda County—Fleming Point, Berkeley Marina: San Mateo County—Coyote Point, western terminus San Mateo Bridge.