Fig. 17

Colony growth form: Similar to A. gemmifera, but longer non-tapering branches.

Axial corallites: Outer diameter 4.3 mm; inner diameter 1.4 mm; prominently large and dome shaped; two cycles of smooth-edged septa are clearly visible; incipient axial corallites are frequent proximally on branch.

Radial corallites: Short, tubular with dimidiate opening and thickened lower wall; generally arranged in rows; their size only increases slightly towards the base of branches; primary septa are small and dentate, while secondary are absent or incomplete, just visible as points.

Coenosteum: Reticulate or lines of elaborated laterally flattened spinules on corallites and coarse reticulate with scattered spinules in intercorallite areas.

Remarks: A. humilis is distinguished from other species of the group by its thinner terete branches and characteristically large axial corallite. (see Wallace 1999; Veron 2000).