Holotype: NTM C12957; C12629; C12630 Holotype, infertile colony on red algae alcohol preserved; microslides from holotype, South of Fish Reef, Grose Islands, Beagle Gulf, Northern Territory, Australia [12°27.90'S, 130°26.70'E].
Named for the two internal submarginal shelves.
Stems simple, monosiphonic, to 10 mm high, arising from a flat, strap-like hydrorhiza; in flexures in perisarc not reaching coenosarc canal. Proximal stem region short, ending in a strong hinge joint, stem internodes thereafter thecate, nodes slender, a transverse collar-like constriction but sometimes with additional hinge joint; infrathecal part widening from node to base of hydrothecae, suprathecal part short.
Hydrothecae biseriate, in contact on front of stem, separated behind, facing slightly forward and outward; base of hydrotheca flat, body tubular, narrowing to margin, adnate adcauline wall parallel to cauline axis, free wall at an angle of 60° to cauline axis, a little shorter than adnate wall, straight to slightly convex, two small spurs pointing downward into infrathecal part at base of adcauline wall; abcauline wall gently concave, slightly tumid near the base with a small submarginal thickening below hydrothecal margin; in older hydrothecae two prominent submarginal shelves project inwards into the hydrotheca from adcauline and abcauline walls. Perisarc of hydrotheca moderately thick. Margin almost elliptical to subquadrate with two broad lobate cusps midway between adcauline and abcauline walls and a rounded, low adcauline cusp. Operculum pyramidal, low, moderately thick, abcauline valve the larger, adcauline flap divided into two; some margins untidily renovated by regenerated opercular valves.
Hydranth stout, with c. 10 tentacles; no abcauline caecum.
Colour: Colourless.
Northern Australia (Northern Territory).
Subtidal, epiphytic on algae.