Shell small, rather thin, either oval or narrow and parallel-sided; elevated, apex subcentral, a trifle recurved ; surface nearly smooth, but showing very fine radiating striae under a lens. Color pure white, sometimes immaculate, but usually having 6 or 7 wide brown rays, which usually do not extend to either apex or basal margin. There is almost always a brown spot just behind the apex. Inside pure white.

Shell small (<8 mm in length), high profile, white with brown rays radiating from apex and small brown apical spot. Aperture oval or laterally compressed. Radula similar to Acmaea mitra , but with more complex lateral plate morphology and uncini. Animal lacks pigmentation. Low intertidal to subtidal, oval form associated with encrusting coralline algae, compressed form with branching coralline algae.

"They present nearly every variation in form, from wide, oval and nearly flat, to narrow, triangular, high and very compressed. The extreme apex is almost always black. It is usually furnished with a few dark brown stripes, radiating from near the apex but seldom reaching the margin in adult specimens. These however are wanting in some specimens. In all its forms it is a well marked species and cannot be united with any now known from the California coast. Dr. Carpenter proposes to rename this form specifically " casta" and to apply the term triangularis to the compressed variety only; it is doubtful, however, if such a course would be admissible, as every transition in form can be observed in a very few specimens.” (Dall.)
Typical triangularis measures: length 6,75, breadth 3, alt. 4 mm.

Port Dick, Alaska (59° 171^) to Santa Barbara, California (34°30'N