Belknapchiton opiparus (Iredale & Hull, 1925)

(Figures 15–18)

Parachiton opiparus Iredale & Hull, 1925: 345, pl. 39, fig. 20; Cotton & Godfrey 1940: 482, fig. 465.

Leptochiton opiparus; Sirenko 2020: 441, fig. 46E.

Leptochiton (Leptochiton) alveolus; Kaas & Van Belle 1985: 36, (part, not Chiton alveolus M. Sars MS, Loven, 1846). Leptochiton (Leptochiton) belknapi; Kaas & Van Belle 1988: 24, (part, not L. belknapi Dall, 1878).

Type material. Holotype (AM C. 32026).

Type locality. Off Cape Wiles, S. Australia, 34°56’44.3’’S, 135°41’3.3’’E, 180 m.

Material examined. South of Australia, Great Australian Bight: 200 km W of S Eyre Peninsula 35.1406°S, 134.2747°E, 450 m, CMAR Voyage stn SS2008_ 03_069, Lot 069, Sherman sled, CSIRO, 1 spm (TMAG, E 45529) BL 7.5 mm, 06.03.2008 .

Distribution. South Australia, Australian Bight, W of S Eyre Peninsula, 180– 450 m.

Given the very short initial description of this species by Iredale & Hull (1925), I decided to give a more extended description of the present specimen.

Description. Small chiton, BL 7.5 mm. Valves thick, highly arched (elevation ratio 0.57 in valve V), not beaked. Second valve is 1.1 times wider than the fifth. Color of tegmentum white.

Head valve absent. Intermediate valve long, oval, lateral area not raised, anterior margin waved in valves III–VIII, lateral margins rounded, posterior margin convex. Tail valve triangular in outline, mucro postmedian, antemucronal slope convex, postmucronal slope slightly concave.

Tegmentum uniformly sculptured with flattened, hardly separated, quincuncially arranged granules.Each granule with one megalaesthete and four micraesthetes in front, pores of megalaesthete a little larger than micraesthetes.

Articulamentum well developed, apophyses small, widely separated, jugal sinus about twice as wide as apophyses.

Girdle narrow (about 0.3 mm near valve V), dorsally covered with obtusely pointed, slightly bent, wide scales (60 x 40 μm) with six–eight distinct longitudinal riblets. Two kinds of flattened marginal needles: obtusely pointed, with ribs (115 x 18 μm), and sharply pointed, smooth (100 x 18 μm). Girdle ventrally covered with elongate, smooth, bluntly pointed scales (one row near margin) (60 x 30 μm), and sharply pointed, smooth scales (50 x 19 μm) in other part of hyponotum.

Radula of studied specimen 4.2 mm long with 26 transverse rows of mature teeth. Central tooth wide, first lateral teeth narrow with small blade, major lateral teeth with unidentate sharp cusps. Distal part of third uncinal teeth rounded.

Remarks. The examined specimen was collected in the Great Australian Bight, not far from Cape Wiles, the type locality of this species. All features of the newly found specimen coincide with those of the holotype, therefore, this is the second find of this rare species. Belknapchiton opiparus superficially resembles most species of the genus, but differs from them by the width of the second valve. Unlike other species of this genus, B. opiparus has a second valve that is wider than all other valves. In addition, some of the marginal needles are ribbed in this species. Belknapchiton opiparus differs from B. okamurai (Saito, 2001), B. bergenhayni (Saito, 2011), and B. fijiensis (Sirenko, 2016), which also have ribbed marginal needles by having five pores of aesthetes in granules of the central area (vs. 7–13 pores in the mentioned species).