Leptochiton longigranum Sirenko 2020, n. sp.

Leptochiton longigranum n. sp.

(Figs. 24–27)

Type material. Holotype (MNHN-IM-2013-67032), a damaged specimen (tail valve was missing), now disarticulated, consisting of valves I, II, IV, V, part of perinotum and radula, mount of part of perinotum and radula mounted on SEM-stub and vial with other valves.

Type locality. Philippines, Balicasag Id., 9°31.2’N, 123°40.7’E, 80–90 m (PANGLAO 2004, stn L42, 02.07.2004).

Etymology. From the Latin longus, long, and granum, granule, referring to the several long granules in posterior margin of intermediate valves in this species.

Material examined. The holotype.

Distribution. Known only from the holotype.

Diagnosis. Small, shell rounded. Tegmentum sculptured with well raised, round or oval granules arranged in quincunx, at the posterior margin of the lateral areas, there are six or more protuberances standing up about four times the height of the larger granules. Dorsal scales pointed, with 4–6 longitudinal ribs. Central tooth of radula short with sharp base, major lateral tooth with bidentate cusp, inner denticle much smaller than outer one. Four gills on each side extending from valve VII to near anus.

Description. Small, BL 2.5 mm (probably adult), oval, valves only little elevated, rounded, not beaked. Color of tegmentum dirty white.

Head valve semicircular. Intermediate valves rectangular, anterior and posterior margin nearly straight, side margins rounded, lateral areas not raised.

Tegmentum sculptured with well raised, round or oval granules arranged in quincunx, granules of central area flattened, those of other areas convex.At the posterior margin of the lateral areas, there are six or more protuberances standing up about four times the height of the larger granules. Each granule with one megalaesthete and seven to nine micraesthetes, all pores arranged at the periphery of flattened top of granule.

Articulamentum white, weakly developed, apophyses broadly rounded, triangular.

Girdle very narrow, densely covered with small, imbricating, conical scales (50 x 26 µm), sometimes truncated at the top, always marked with 4–6 sharp ribs, which are twice less than interstices; intersegmental needles very long and smooth (350 x 17 µm). Margin armed with two different kinds of elements: 1) few long smooth needles similar to the intersegmental ones (350 x 17 µm), 2) dorsally finely ribbed (6–7 ribs) sharply pointed scales (80 x 18 μm) Ventral side near margin with one row of small (50–60 x 20 μm), oval, distally obtusely pointed scales with four short ribs in upper half; other ventral scales smooth, pointed (46 x 15 µm).

Radula 0.7 mm long with 25 transverse rows of mature teeth. Central tooth short, rectangular, with sharp base, first lateral tooth short, triangular, without a blade, major lateral tooth with bidentate cusp, inner denticle much smaller than outer one.

There are 4 gills per side arranged from valve VII to anus.

Remarks. The new species can be easily recognized by the protuberances at the posterior margin of the intermediate valves. Leptochiton longigranum n. sp. resembles Leptochiton finlayi (Ashby, 1929) from New Zealand which also has protuberances at the posterior margin of the lateral areas, but differs from by having granules arranged in quincunx (vs. granules in head valve, central areas of intermediate valves and antemucronal area of tail valve arranged in longitudinal rows in L. finlayi) and four gills per side (vs. nine gills in L. finlayi).