Fissurella (Cremides) microtrema Sowerby, 1835 (Synonyms: F. chlorotrema and humilis Menke, 1847; F. rugosa of authors, not of Sowerby, 1835). Variable, with an irregular outline, depressed, narrow anteriorly usually with three anterior ribs that are stronger than the rest, projecting at the margin; the entire shell with rough and scaly irregular ribbing. Orifice relatively small, tripartite in young specimens, oval in mature shells. Interior greenish white, the callus area outlined with gray or yellow-orange. Color pattern variegated, mostly reddish or greenish-rayed, major ribs light-colored. On exposed rocks, usually covered with tufts of algae. Length, 32 mm; width, 22 mm; height, 8 mm. Cape San Lucas and the southern part of the Gulf of California through Mazatlán, Mexico, and south to Salinas, Ecuador.