Dibunostoma reversum (Harmer, 1926)

(Fig. 1)

Calpensia reversa Harmer, 1926: 309, pl. 20, figs 13–16. Dibunostoma reversum: Cheetham 1963: 52, fig. 27a; Soule et al. 1991a: 462; Soule et al. 1992: 7.? Thalamotreptos reversus: Soule et al. 1991a: 460, pl. 4, fig. 4.

Material examined. Two colonies (NIBRIV0000805879, MBRBKW1), Munseom Island, Jeju, 20 m, collected by Ho Jin Yang.

Description. Colony encrusting, multiserial, unilaminar, unpigmented, up to 25 mm across. Autozooids more or less subrectangular, longer than wide, the raised lateral margins parallel, usually slightly converging proximad, less often diverging. Cryptocyst sunken below level of zooidal rim, rising towards proximal rim of opesia-orifice from level of opesiules; cryptocystal surface densely granular and minutely pitted proximal to opesiules, not pitted where it rises up to proximal orificial rim. Opesiules paired, conspicuous subrounded to suboval.

Opesia as wide as long or wider; orifice with extensive sloping oral shelf and rounded rim distally, narrowing to short, straight proximal margin. No oral spines. A conspicuous pair of mamilliform knobs flanking orifice.

Avicularium interzooidal, small, short, asymmetrical, at bifurcation of zooid row, directed obliquely proximally and indenting orificial rim; rostrum shaped like equilateral triangle; mandibular pivots blunt, weakly developed; proximal cryptocyst granular.

Ooecia unknown. Ancestrula not seen.

Measurements. ZL 780–1036 (895) µm; ZW 436–528 (472) µm; OrL 93–132 (117) µm; OrW 162–207 (189) µm; AvL 272–315 (296) µm; AvW 235–267 (249) µm.

Remarks. This species is notable for its short, reversed-orientation interzooidal avicularia. It was questionably included in their new genus Thalamotreptos by Soule et al. (1991a), who equivocated on the scope of Dibunostoma Cheetham, 1963. A key character of Thalamotreptos was the proximally or proximolaterally directed avicularium. Two of the species included in Thalamotreptos have a small opesial foramen separated from the avicularian rostrum by a cryptocystal bridge (which may not function as a pivot bar), lacking in D. reversum. Conceivably this bridge was lost in D. reversum or Thalamotreptos is unrelated and may be retained as a separate genus.

Distribution. Sri Lanka: locality unknown; Korea: Jeju Island, 20 m depth.