Ischnochiton tridentatus Pilsbry, 1893

(Figure 2B)

Ischnochiton (Radsiella) tridentatus Pilsbry, 1893: 35–39 . Chresonymy and synonymy in Kaas & Van Belle (1990).

Type material. Lectotype, designated by Kaas & Van Belle (1990), ANSP 66324.

Type locality. La Paz, Baja California Sur, México (Kaas &Van Belle 1990) .

Material examined. Seventy– six specimens, BL 7.3–20.2 mm.

Habitat. In the superficial intertidal on medium–sized and small rocks buried in sand. Specimens were collected from numerous loose rocks with dense patches of globular thalli (maximal diameter of 20 cm) of the alga Dictyosphaeria Decaisne, 1842 .

Remarks. Ischnochiton tridentatus has a sculpture of concentric zigzag lines and variable tegmentum color of light olive, dark green and gray (Fig. 2B).

Kaas & Van Belle (1990) reported the distribution of this species throughout the Gulf of California, including Mazatlán. However, recent studies have shown that I. tridentatus is absent in Mazatlán, while it is abundant at La Paz, Baja California Sur (García-Ríos & Álvarez-Ruiz 2007). In the present study, it was only registered at Bahía San Carlos, Sonora. The maximal size measured is 26.1 mm, coinciding with previous reports by Kaas & Van Belle (1990).

Ischnochiton tridentatus is endemic to the Gulf of California. In our surveys, it was common to find small (maybe juveniles) and adult specimens cohabiting at beaches with low wave energy.