Acanthochitona achates (Gould, 1859) Figs 21–23, 44 A Chiton (Acanthochaetes) achates Gould 1859: 165. Acanthochiton sagamicus Bergenhayn 1933: 43, pl.1, fig.14. Acanthochiton achates: Is. Taki, 1938: 360 –362, pl. 15, fig. 2, pl. 23, figs 7–11, pl. 24, figs 3–7. Acanthochitona achates: Kaas & Van Belle, 1980: 1; 1998: 11; Saito, 1995: 109; 1998: 161; 2000a: 102; 2000b: 23, pl. 11, fig. 9; 2001: 200, fig. 17; 2006a: 217; Slieker, 2000: 100, pl. 38, fig. 26. Type material. Lectotype, MCZ 169031. Type locality. Kikaia and Hakodadi (= Hakodate) Bay, Hokkaido, Japan. Material examined. Central Vietnam, near Da Nang, 16°11.802´N, 108°10.593´E, intertidal, stones, (on barnacles), 6 spms, BL 3–19 mm, 16.05.2012, leg. O. Savinkin. Description. Animal medium in size, oval, with small, low, round-backed valves. Color of tegmentum variable, often brown with whitish streaks along jugum, bluish green patches on lateropleural areas. Color of girdle also variable, brown, dark yellowish brown or yellowish green, sometimes with darker or lighter bands or patches. Head valve more than semicircular, with five faint radial undulations; posterior margin almost straight; anterior slope convex. Intermediate valves roughly trapezoidal, beaked, round-backed; lateral areas hardly separable; side slopes slightly convex; jugum wedge-shaped, with fine longitudinal grooves. Tail valve small, elliptical to trapezoidal, wider than long, with central mucro; posterior slope convex. Tegmentum sculptured with oval to elongate oval pustules, arranged quincuncially on all tegmental areas except for jugum which has longitudinal riblets separated by narrow grooves. Each pustule with one central macraesthete pore, 0–2 micraesthete pore on pre-pustular slope. No aesthete pores on tegmental plain. Articulamentum well developed, thick, bluish white. Apophyses separated by wide sinus; anterolateral margin weakly round in intermediate valves, truncate in tail valve. Insertion plate long, smooth, vertically projected in tail valve. Slit formula 5/1/2. Slits narrow, short. Girdle wide, about 3.1 mm near valve IV (width 4.3 mm) in studied specimen (BL 15 mm). Dorsal side of girdle of studied specimen (BL 15 mm) densely covered with straight, finely striate, pointed spicules of two distinct sizes: up to 260 µm x 40 µm and 150 µm x 30 µm. Sutural tufts prominent, consist of up to 70 large needles, up to 1200 µm x 42 µm. Marginal needles similar to larger perinotal spicules, but longer and smooth, 600 µm x 30 µm. Ventral spicules small, slender, flat, weakly angulated on surface, 100 µm x 19 µm. Gills extending from valve VII to valve IV, composed of 20 ctenidia on each side in studied specimen (BL 15 mm), Radula of studied specimen (BL 15 mm) 5.5 mm long with 33 transverse rows of mature teeth. Central tooth cup-shaped with weakly bilobed blade, strongly keeled at baseal half. First lateral (centro-lateral) tooth with thick, nodulous antero-dorsal corner. Major lateral tooth with tricuspid cap; denticles blunt at tip; central one longer than others. Distribution. Japanese coast from southern Hokkaido to Okinawa Island; Korean coast; East China Sea Coast of China; northern Vietnam, intertidal. Remarks. Type material of Chiton achates Gould, 1859 (MCZ 169031) comprises three specimens. Among which, two specimens are Acanthochitona achates, while another is Leptoplax doederleini. Gould (1859) gave two localities of the specimens: Kikaia and Hakodadi Bay (Hokkaido). If Kikaia means Kikai-jima Island (or Kikai-gashima Island), Ryukyu Islands, approximately 28° 29´N, 130° 00´E, this appears to be the locality of the specimen of L. doederleini as this species has been known only from the Pacific coast of central Japan and southwards. This species is distinct from its Indo-West Pacific congeners by having a well defined wedge-shaped jugum with fine longitudinal grooves, single macraesthete pore on each pustule of the tegmentum, lack of the aesthete pores on the tegmental plain, spiculose girdle which comprises pointed, finely striated spicules of two distinct sizes. “ Notoplax doederleini ” (not of Thiele, 1909) reported from Hong Kong (Bullock & Harper 1994) appears to be identical with this species. This is the first record of this species from the Vietnamese coast.

Published as part of Sirenko, Boris I. & Saito, Hiroshi, 2017, New species of chitons of the superfamily Cryptoplacoidea (Mollusca: Polyplacophora) from Vietnamese waters, pp. 451-506 in Zootaxa 4299 (4) on pages 474-478, DOI: 10.11646/zootaxa.4299.4.1, http://zenodo.org/record/836971