(Figures 2D, 5A–E)

Ischnochiton mariposa Bartsch MS, Dall, 1909: 499–516. Chresonymy and synonymy in Kaas & Van Belle (1990).

Type material. Syntype, USNM 58865.

Type locality. Gulf of California (Kaas & Van Belle 1990).

Material examined. 162 specimens, BL 5.2–14.3 mm.

Habitat. In the intertidal during low tides, adhered to medium–sized and small rocks buried in sand. It was collected from loose rocks with crusty brown and red algae patches with numerous globular thalli of Dictyosphaeria Decaisne, 1842.

Remarks. Stenoplax mariposa has a sculpture of irregular flat nodules (Figs 5A–C); central areas with longitudinal fine grooves; lateral areas indicated by a change of the sculpture arrangement (Fig. 5B). Dorsal scales of girdle subrectangular shaped, somewhat bent with 22–32 longitudinal grooves distally (Fig. 5D). Radula with tricuspid major lateral and narrow, short central tooth (Fig. 5E).

Stenoplax mariposa, along with S. petaloides Gould, 1846 from the Hawaiian Islands, S. rugulata (G.B. Sowerby I, 1832) with a distribution range from Nicaragua to Ecuador (including the Galapagos Islands) all together form a species complex (Reyes-Gómez et al. 2023). This complex is characterized by their small size (BL 8–18 mm) and by having a tegmentum with a sculpture of longitudinal broken grooves in the central areas; lateral areas not raised, with irregular nodules or fine grooves; girdle scales twice or thrice as wide as long, with numerous longitudinal ribs; radula with narrow and elongated central tooth and the major lateral tooth with a tricuspid head or cusp. At a first glance, these species are very similar to each other, especially due to the pattern of the tegmentum color (purple, olive green, light green, brown, cream, and white) and characteristic turquoise dots located in the jugal area of the intermediate valves.

Stenoplax mariposa differs from its congeners by displaying a sculpture of irregular, flat nodules on the lateral areas, the head, and on the anteromucronal area of the tail valves (Figs. 5A–C). It displays longitudinal narrow, broken ribs that are somewhat elevated, covering the entire central area (Fig. 5B).