Microporina okadai Silén, 1941

(Fig. 27; Table 25)

Microporina okadai Silén, 1941: 68, figs 79–82, pl. 4, fig. 13, 14. Material examined. Holotype by original designation UPSZTY 2468, Okinose, Sagami, Japan; depth 150–600 m. Leg. Prof. S. Bock 1914.

Description. Colony erect, jointed, dichotomously branching (Fig. 27A); internodes straight, quadrangular in cross-section, quadriserial (Fig. 27D).

Autozooids elongate-rectangular (mean L/ W 3.70), proximal margin concave, distal margin convex, arranged in alternating back-to-back series; cryptocyst forming a raised mural rim (50–60 µm wide), minutely granular with granules (5 µm or less in diameter) regularly aligned forming multiple concentrical rows, and modified to follow the outline of the avicularium at the distal edge (Fig. 27E); frontal cryptocyst immersed and flat with sparse granules, slightly coarser than those of the mural rim (8–10 µm), and unevenly distributed circular pseudopores (10–15 µm in diameter) (Fig. 27B); a pair of lateral opesiules at a short distance (c. 40–50 µm) from the proximal margin of the orifice (Fig. 27B, see arrows).

Orifice semielliptical with straight or concave proximal margin, wider than long, outlined by a raised rim (Fig. 27B).

Avicularium adventitious, single, placed distally to most autozooids, oval (Fig. 27D–F); rostrum acutely triangular, raised, proximally directed; crossbar complete; mandible triangular with downward hooked tip (Fig. 27C, see arrow).

Ovicells absent.

Remarks. The autozooid illustrated in Fig. 27B was the only one without a frontal membrane, in which lateral symmetrical depressions adjacent to the mural rim and at a short distance from the proximal margin of the orifice were observed. These structures were, therefore, interpreted as the ‘opesiulae’ described and drawn in Silén (1941, fig. 81). SEM images of an internode with well-defined opesiules are available from Arakawa (2016, fig. 9B).

Of the 12 species currently assigned to Microporina more than half are from the Pleistocene to Recent of Japan (Bock 2023). Based on the species comparison of Arakawa (2023, table 1), among the Japanese Microporina, M. okadai has the narrowest internodes and longest autozooids.