Fig. 13

Colony growth form: Digitate or corymbose with thick tapering; almost conical branches; incipient axial corallites occur near base of branches.

Axial corallites: Outer diameter 2.7–3.5 mm; inner diameter 1.0– 1.1 mm; short but conspicuous; thick walled with a small opening; septa are reduced and smooth edged but clearly present in two cycles.

Radial corallites: Dimorphic, usually in rows; larger ones are tubular with dimidiate opening (or completely dimidiate) and may become slightly nariform at branch extremity; typically, these increase in size down the branch and have thickened lower wall; smaller radials are sub-immersed; septal development is rudimentary and dentate.

Coenosteum: Irregular costae on corallites and coarsely reticulate with scattered spinules in intercorallite area.

Remarks: A. gemmifera and other species of the A. humilis group can be difficult to distinguish as noted in the literature. In Wallace and Wolstenholme (1998), A. gemmifera is “ distinguished by the consistent presence of immersed as well as longer radial corallites throughout a branch in A. gemmifera ”. We have made the distinction on this basis and because the size of radial corallites clearly increases down the branch (Veron & Wallace 1984).