Corbulella boninensis (Silén, 1941)

(Fig. 5; Table 6)

Pyrulella boninensis Silén, 1941: 26, figs 21–24.

Crassimarginatella (Corbulella) boninensis: Gordon, 1984: 30.

Corbulella boninensis: Yang et al., 2018: 497, figs 6, 7.

Material examined. Lectotype (designated here) UPSZTY 2470 D, the best preserved specimen among the syntypes UPSZTY 2470 A–D, Bonin Islands (Ogasawara), east of Chichijima, Japan; depth 100–135 m. Leg. Prof. S. Bock 1914 . Paralectotypes: the three remaining specimens.

Description. Colony encrusting, multiserial, unilaminar, fan-shaped (Fig. 5A).

Autozooids oval to club-shaped if with a proximal, extensive gymnocyst (Fig. 5A, B, D), longer than wide (mean L/ W 1.86), distinct, separated by deep grooves, alternating in radial series. Gymnocyst highly variable in width proximally (i.e. 40–170 µm), much narrower laterally and obscured by adjacent zooids, smooth; cryptocyst outlined by a raised beaded rim (Fig. 5E), sloping towards the opesia, more extensive proximally (40–115 µm), tapering laterally (20–35 µm), disappearing distally, coarsely granular with granules 8–12 µm in diameter (Fig. 5B, E).

Opesia oval, occupying two-thirds to half zooidal length (mean OpL/ZL 0.63) (Fig. 5B, D); eight lateral opesial spines (Fig. 5C, F), four per side, thin, 5–8 µm wide, 90–100 µm long, widely spaced, curved over the aperture but not meeting in the midline, indenting the cryptocyst at the base (Fig. 5E); 3–7 shorter spines placed distal and distolateral to the orifice, 50–70 µm long, 10–15 µm in diameter (Fig. 5B–D); orificial opening transversely Dshaped, 80–90 × 125–150 µm.

Avicularia vicarious, similar in size to autozooids, broadly figure-eight-shaped (Fig. 5A see arrow, G); rostrum spatulate with raised distal and finely denticulate distolateral margins; at least two pairs of opesial spines, two per side, placed at about mid-length immediately below the raised margin of the rostrum; at least two distolateral spines; mandible semielliptical.

Ovicells prominent, cap-like (Fig. 5C, F, G), resting on the proximal gymnocyst of the distal zooid; ectoooecium smooth, partially calcified, leaving an arc-shaped frontal fenestra exposing a smooth endooecium; the distal rim of the fenestra typically notched centrally forming a small umbo (Fig. 5D, F, G); only two distolateral oral spines visible in ovicellate zooids (Fig. 5F, G).

A single kenozooid observed, 205 × 130 µm, pear-shaped with smooth gymnocyst laterally and coarsely granular, depressed cryptocyst frontally, an elliptical opening 80 × 30 µm placed centrally (Fig. 5H).

Remarks. Over the years, following Hastings (1945), Brown (1952) and Harmelin (1973), species of the genus Pyrulella Harmer, 1926 were either included in Valdemunitella Canu, 1900 or in Crassimarginatella Canu, 1900 . The genus Corbulella was first introduced as a subgenus of Crassimarginatella (Gordon 1984) and regarded as such until Tilbrook et al. (2001) recommended to treat it as a full genus owing to the high number of species attributed to it and its long Cenozoic range. Corbulella can be distinguished from Crassimarginatella by the presence of vicarious avicularia with toothed rostra and spines bordering the opesia (Gordon 1984).

Genus Cranosina Canu & Bassler, 1933