Inventory of chiton species (Polyplacophora) from the rocky intertidal of the Northern Gulf of California, with an illustrated taxonomic key Author Reyes-Gómez, Adriana Doctorado en Biosistemática y Manejo de Recursos Naturales y Agrícolas. Author Vargas-Ponce, Ofelia Departamento de Botánica y Zoología. Author Galván-Villa, Cristian Departamento de Ecología Aplicada, Centro Universitario de Ciencias Biológicas y Agropecuarias, Universidad de Guadalajara, Camino Ing. Ramón Padilla Sánchez 2100, Zapopan, Jalisco, 45200, México. Author Salgado-Barragán, José Laboratorio de Invertebrados Bentónicos, Unidad Académica Mazatlán, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, México. Author Esqueda-González, Ma. Del Carmen Departamento de Ecología Aplicada, Centro Universitario de Ciencias Biológicas y Agropecuarias, Universidad de Guadalajara, Camino Ing. Ramón Padilla Sánchez 2100, Zapopan, Jalisco, 45200, México. Author Ríos-Jara, Eduardo Departamento de Ecología Aplicada, Centro Universitario de Ciencias Biológicas y Agropecuarias, Universidad de Guadalajara, Camino Ing. Ramón Padilla Sánchez 2100, Zapopan, Jalisco, 45200, México. text Zootaxa 2023 2023-05-26 5296 2 147 178 http://dx.doi.org/10.11646/zootaxa.5296.2.2 journal article 10.11646/zootaxa.5296.2.2 1175-5326 7973065 624273A6-3028-42C2-ABE2-A18BBF828156 Ischnochiton guatemalensis Thiele, 1909 ( Figures 2A , 4A–I ) Ischnochiton guatemalensis Thiele, 1909:51–56 . Chresonymy and synonymy in Kaas & Van Belle (1990) . Type material. Lectotype ZMB Moll. 102027 ( Kilias 1995 ). Type locality. Champerico , Guatemala or Acajutla, El Salvador ( Kaas & Van Belle 1990 ). Material examined. Two specimens, BL 6.8–8.1 mm . Habitat. Under big, semi–humid rocks. The specimens were found on rocks buried in sand with brown and red crusty algae patches. Remarks. The examination of Ischnochiton guatemalensis revealed microgranular sculpture ( Figs. 4A–C ) of valves, lateral areas somewhat raised ( Fig. 4B ). Each microgranule with 16–19 microaesthetes ( Figs. 4D–E ). Girdle dorsal scales, somewhat bent with 7–10 longitudinal ribs which converge distally ( Figs. 4F–G ). Radula with bicuspid major lateral and narrow central tooth ( Figs. 4H–I ). According to Kaas &Van Belle (1990) , I . guatemalensis shows a continuous distribution in the intertidal, from the Gulf of California to Guatemala . Nevertheless, it has not been registered since Kaas &Van Belle (1990) reported it in their monograph of worldwide living chitons, nor in recent studies such as the ones performed about the fauna from Mazatlán, Sinaloa (Ortiz–Arellano & Flores –Campaña 2008), the Mexican Tropical Pacific (MTP) (Jalisco, Guerrero and Oaxaca) (Reyes–Gómez et al . 2010; Flores-Garza et al . 2012 ; Galeana–Rebolledo et al . 2014; ReyesGómez et al . 2022) and the coast of El Salvador (García-Ríos et al . 2007). In the present study, we only obtained two individuals from both sites (Bahía la Choya, Puerto Peñasco), indicating that it may be a rare species (with respect to its abundance and occurrence) in the Northern Gulf of California. The absence of reports of I . guatemalensis in some regions of the Gulf and the MTP is also reflected in the lack of representative material of this species in the malacological collections. The report of this species from the northern Gulf of California represents the rediscovery of I . guatemalensis in this region after Kaas & Van Belle's report (1990).