Kaasichiton ionensis n. sp.

(Figures 9–15, 22)

Type material. Holotype (ZIN 2462) and paratype (ZIN 2463) now disarticulated consisting of SEM stub of valves I, II, V, VIII, part of perinotum and radula, mount of part of perinotum and radula and vial with other valves.

Type locality. The Sea of Okhotsk, Iona Island, 43 m.

Etymology. Named after the Iona Island.

Material examined. Six specimens in total. The Sea of Okhotsk, Iona Island, R / V Poseidon, sample 85, grab, 43 m, stones, holotype (ZIN 2462), BL 3.8 mm, and paratype (ZIN 2475), BL 3.0 mm, 21.07.1978; sample 96, SCUBA, 17 m, rocks, 2 paratypes (ZIN 2463) BL 4.0 mm, 21.07.1978; sample 98, grab, 43 m, stones, 2 paratypes, BL 3.0– 4.8 mm, 22.07.1978.

Distribution. Off Iona Island, the Sea of Okhotsk, 17– 43 m.

Diagnosis. Animal small, elevated, valves subcarinated, mucro of tail valve posterior, antemucronal area noticeably convex, postmucronal slope decidedly concave near mucro and then convex; tegmentum sculptured by small, rare, very low granules, arranged more or less in longitudinal rows in central areas of intermediate valves and antemucronal area of tail valve, and in a random manner in other areas of valves, each granule with one megalaesthete; dorsal scales narrow, sculptured with 3–6 vague riblets on dorsal side, marginal needles with 4–5 vague riblets; central teeth of radula short with large rhomboidal base, narrowing in front to a small blade, first lateral tooth somewhat wing-shaped, with small blade, major lateral teeth with thin tridentate cusp, median denticle obtuse pointed, much longer than others; six gills on each side.

Description. Holotype (BL 3.8 x 2.2 mm) elongate oval, valves subcarinated, moderately elevated (dorsal elevation 0.45), not beaked, side slope convex, mucro slightly damaged, tegmentum white in color.

Head valve semicircular, sculptured by small granules arranged in a random manner; intermediate valves rectangular, anterior and posterior margins slightly convex especially in valve II, lateral area not raised, central area sculptured by small, rare, very low granules distributed in about 20 obsolete longitudinal rows; tail valve narrower than head valve, a wide flattened posterior mucro shifted to the postmucronal area, antemucronal area noticeably convex, postmucronal slope decidedly concave near mucro and than convex, antemucronal area sculptured like central area of intermediate valves, postmucronal area sculptured like head valve. Each granule contains one megalaesthete.

Articulamentum moderately developed, white, apophyses small, widely separated from each other, trapezoidal.

Girdle narrow, dorsally covered with elongated, obtuse pointed, scales (65–70 x 25 µm) sculptured with 3–5 vague riblets and interspersed with, randomly disposed long needles (150 x 21 µm). Intersegmental areas with similar needles. Marginal needles of the same type as sutural ones, but a little smaller (120 x 21 µm). Most needles with 4–5 vague riblets. Ventral side covered with imbricated, flat, elongate, obtuse pointed scales (70 x 20 µm).

Holotype with six gills on each side, extending from valve VI to near anus.

Radula 2.0 mm long, with 64 transverse rows of mature teeth, central teeth of radula short with large rhomboidal base, narrowing in front to a small blade, first lateral teeth somewhat wing-shaped, with small blade, major lateral teeth with thin tridentate cusp, median denticle obtuse pointed, much longer than others.

Remarks. Kaasichiton ionensis n. sp. is closest in sculpture of the tegmentum to K. lukini and K. incubatus. The new species differs from the K. lukini in having a tridentate cusp of the major lateral radular tooth (vs bidentate cusp in K. lukini), an expanded rhomboidal base of the central tooth, and about half the number of rows of mature radular teeth (60–64 versus 123 rows in K. lukini). The new species differs from K. incubatus by having long, ribbed scales on the perinotum (vs short scales in K. incubatus), a tridentate cusp of the major lateral radular tooth (vs unidentate cusp in K. incubatus), and rhomboidal enlargement of the base of the central tooth (vs without enlargement of the base in K. incubatus).

Kaasichiton ionensis n. sp. differs from K. kamchatkensis n. sp. in having a longer first lateral radular tooth that is longer than the central one (vs both teeth having about the same length in K. kamchatkensis n. sp.), a tridentate cusp of the major lateral radular tooth (vs bidentate cusp in K. kamchatkensis n. sp.), and rhomboidal enlargement of the base of the central tooth (without enlargement of the base in K. kamchatkensis n. sp.).

The new species often occurs together with L. assimilis Thiele, 1909 from which it can easily be distinguished by the sculpture of the tegmentum, very narrow perinotum, posterior mucro, tridentate cusp of major lateral radular teeth, and very different shape of central and first lateral radular teeth.