Retevirgula triangulata (Silén, 1941)

(Fig. 9; Table 10)

Pyrulella tubulata var. triangulata Silén, 1941: 28, figs 25, 26.

Retevirgula tubulata var. triangularis [sic]: Mawatari & Mawatari, 1980: 89, fig. 31.

Retevirgula triangulata: Min et al., 2017: 475.

Material examined. Holotype by original designation UPSZTY 2471, Bonin Islands (Ogasawara), Takino Ura, Japan; depth 45–60 m. Leg. Prof. S. Bock 1914.

Description. Colony encrusting, multiserial, unilaminar (Fig. 9A, B).

Autozooids oval to almost parallel-sided elliptical (Fig. 9B, F), twice as long as wide (mean L/ W 1.96), quincuncially or irregularly arranged, disjunct with 8–14 tubular connections (30–90 µm wide) to adjacent autozooids and heterozooids (both avicularia and kenozooids); tubular connections more visible in certain portions of the colony, leaving large (70–225 × 40–110 µm), irregularly elliptical lacunae between them (Fig. 9B, E). Gymnocyst smooth, more extensive proximally (80–90 µm), sloping steeply laterally; cryptocyst forming a very narrow (10–15 µm) coarsely beaded rim around the opesia, indented by periopesial spines (Fig. 9B, C, F).

Opesia oval or elliptical, somewhat mirroring autozooidal shape, occupying most of the frontal surface (mean OpL/ZL 0.77), encircled by 14–18 articulated spines, 20–30 µm in diameter, 140–230 µm long, the two distalmost pairs more erect, the remaining pairs overarching the frontal membrane (Fig. 9A).

Avicularia and kenozooids interzooidal, sparse (Fig. 9A–D) or forming clusters (Fig. 9E), the shape, size and convexity of the cystid/zooid variable. Avicularia elliptical, slightly constricted at about mid-length, an extremely narrow rim of beaded cryptocyst outlining its proximal half (Fig. 9C, D); mandible and rostrum semicircular, the latter raised and directed distally or distolaterally to either side; pivotal bars or condyles absent. Kenozooids with circular openings surrounded by a rim of beaded cryptocyst (10–15 µm wide) and 5–6 erect spines, 10–20 µm in diameter, 100–150 µm long, not indenting the cryptocyst but placed at a short distance (10–25 µm) from it (Fig. 9E–H, see arrows).

Ovicells globular, convex; ooecium smooth, ectooecium partially calcified, with a plectrum-shaped or teardrop-shaped fenestra, placed medially (Fig. 9A, C, F).

Remarks. Min et al. (2017) pointed out that in the description of this species neither Mawatari & Mawatari (1980) nor Silén (1941) mentioned the presence of spinose interzooidal kenozooids, which were indeed observed in the holotype (see Fig. 9E–H).